NORTHWESTERN
UNIVERSITY
LIBRARY
EVANSTON
ILLINOIS
H ö-nJL
THE STUDENT'S HANDBOOK OF
PHYSICAL GEOLOGY.
VIEW OF THE DREI ZINNEN (DOLOMITE PEAKS) FROM NEAR LANDRO ;
Showing the forms developed by the weathering of vertical joints.
THE STUDENT'S HANDBOOK
OF
PHYSICAL GEOLOGY.
BY
A. J. JUKES-BEOWNE, B.A., F.G.S.,
OF THE GEOLOGICAL Sl'RVEY OF ENGLAND AND WALES.
With numerous Diagrams and Illustrations.
SECOND EDITION, REVISED.
LONDON:
GEOKGE BELL & SONS, YOEK ST., GOVENT GABDEN,
AND NEW YORK.
1892.
55ú, Z
CHISWICK PRESS C. WHITTÍNGHAM AND LO., TOOKS COURT,
CHANCERY l.ANE.
PEEFACE.
AS stated in the Preface to the First Édition of this
treatise, my aim in writing it was to supply a hand¬
book of Geology at a moderate price, and in a form that
was specially adapted for teaching purposes ; a book with
which every member of a geological class could provide
himself, and one upon which the teacher could readily
build up a superstructure of further geological knowledge.
A text-book of geology must necessarily be chiefly occu¬
pied in presenting known facts and phenomena, and in
describing suitable instances or illustrations of them, but
its value for educational purposes must greatly depend
upon its plan or system of arrangement, and upon the
manner in which its contents are presented to the reader.
The general arrangement in this volume is that which
has always seemed to me the most natural and the best
fitted for the gradual development of the subject. Dry
details respècting minerals and rocks are apt to discourage
the young student if he is obliged to master them before
coming to the descriptive branches of geology.
The arrangement of the matter in each chapter has also
been considered, for tmless the writer or lecturer forms
for himself a clear idea of what he is going to say on each
separate subject, he cannot hope to make it clear to his
readers or hearers.
PhyBiographical Geology now takes rank as a primary
vi
PREFACE,
division of the subject, and its natural place in a text-book
seems to be immediately after the sections on Dynamical
and Structural Geology. I agree with Mr. G. K. Gilbert
of the U. S. Geological Survey, who says, " starting from
geological agencies as data, we may proceed in one direction
to the development of geological history, or in another to
the explanation of terrestrial scenery and topography, and
if the development of the earth's history is the peculiar
theme of geology, it follows that the explanation of topo¬
graphy or physiographical geology is of the nature of an
incidental result,—a sort of corollary to dynamical geology."
This volume includes only the three branches of the
subject mentioned above, those of Palaeontological and
Historical Geology being left to form another volume.
At the time the First Edition was written there were
certain subjects which it was necessary to treat with much
caution, but during the last few years much light has been
thrown on the questions of Metamorphism, Mountain-
building, and the substructure of the Earth's crust, so that
more definite and detailed information can now be placed
before the reader. Other portions of the work also required
fuller treatment, so that the volume has been expanded by
more than a hundred pages, and the number of illustra¬
tions has been correspondingly increased. Indeed so much
has been added or re-written, that this edition is almost a
new book, but the same arrangement of subjects has been
preserved.
In preparing this edition I have spared no pains to
make it a reliable handbook for those branches of the
science which it embraces, and I have to thank many
friends for assisting me in this object. To Professor
Bonney and Kev. O. Fisher my thanks are especially due
for aid and information regarding the subjects which are
particularly their own, and to Professor Bonney I am also
PREFACE.
viî
indebted for many hints and corrections made on the proofs
of the second half of the book, which he was kind enough
to read. For the revision of the chapters on Igneous Eocks
I am responsible, but the alterations have Professor
Bonney's approbation, and most of what was written by
him for the first edition has been incorporated.
Most of the diagrams have been drawn by myself"; of
the other illustrations, some have been taken from Dr.
Mantell's works, and others, including the frontispiece,,
from Professor Bonney's " Alpine Eegions," by permission
of the author and publishers. Additional views have been
obtained for this edition from Messrs. Cassell, and from
the publishers of the " Pictorial Itinerary of North Wales"
(Woodall and Co., Oswestry).
In conclusion, I need only add that I shall feel grateful
for any criticisms or suggestions that may lead to the
improvement of the work, should a third edition be called
for.
A. J. Jukes-Beowne.
March, 1892.
Introduction
CONTENTS.
page
1
PART I.—DYNAMICAL GEOLOGY.
Section I.—Changes produced by the Influence of
Internal or Subterranean Causes.
Chapter I.—The Mass and General Structure of the
Earth 9
The Shape of the Earth and its Consequences. Internal
Temperature. Internal Condition of the Earth.
Chapter II.—Volcanoes 20
General Form and Structure. Phenomena of an Erup¬
tion. Products of Eruption. Dykes. Causes of Volcanic
Action.
Chapter III.—Volcanoes [continued) ...» 35
Examples of Volcanic Action ;—Vesuvius, Lipari Is¬
lands, Etna, Hawaii. Volcanic Islands and Submarine
Volcanoes.
Chapter IV. —Earthquakes 49
Connection with Volcanoes. Nature and Origin. Phe¬
nomena and Effects. Instances of Earthquakes. Sum¬
mary of Surface Effects.
ChjVpter V.—Elevation and Depression of Land . 65
Proofs of Elevation :—Raised Rocks, Shells, and
Beaches ; Old Sea-caves. Proofs of Depression :—Sub¬
merged Forests, Buried Valleys, Fiords. General Con¬
clusion.
x
CONTENTS.
Section II.—Changes produced hy the Agencies which
operate on the Surface of the Earth's Crust.
a. Processes of Disintegration.
page
Chapter VI.—Terrestrial Agencies .... 83
Rain, its Chemical Action, and Mechanical Effects.
Action of Wind. Action of Heat and Cold. Action of
Frost. General Results. Definition of Terms.
Chapter VII.—Underground Circulation of Water. 108
Causes of Springs. Underground Water ; its Chemical
Action—Swallow^oles and Caverns. Mechanical Action
—Landslips.
Chapter VIII. —Fluviatile Agencies. § 1. Rivers . 126
How a River is Formed. Laws of Flow. Transport of
Detritus. Matter in Solution. Quantities of Transported
M.atter. Erosive Power. Vertical and Lateral Erosion.
Rapidity of Erosion. Instances of River-erosion.
Chapter IX.—Fluviatile Agencies. § 2. Glaciers . 151
Formation of Glaciers. Movement. Tfansport of De¬
tritus. Erosive Action. Ice-sheets.
Chapter X.—Marine Agencies 164
Action of Waves. . Formatiqn of Caves, Coves, and
Inlets. Formation of Cliffs. Rapidity of Marine Ero¬
sion. Heligoland. Action of Tidal Currents. Coast-ice.
b. Processes of Reconstruction.
Chapter XI.—Terrestrial Deposits . . . .188
Mechanical Deposits :—Soil, Rainwash, etc. Mountain
Screes and Breccias. Clay with Flints and Red Earth,
Cave-earth. Kaolin. Loess. Blown Sand.
Chemical, Deposits :—Stalagmite, Tufa, Gypsum,
Sinter. Organic Deposits :—Mould. Peat. Mangrove
Swamps.
Chapter XII.—Fluviatile Deposits .... 216
§ 1. Deposits formed by Rivers :—Deposition of De¬
tritus. Gravel Beds and Alluvial Levels. Formation of
Deltas. § 2. Deposits formed by Glaciers :—Moraines.
TiU and Boulder Clay.
Chapter XIII.—Lacustrine Deposits .... 234
Deposits in Fresh-water Lakes, of (1) Mechanical, (2)
Organic, (3) Chemical Origin. Deposits in Saline Lakes.
CONTENTS.
xi
page
Chapter XIV.—Marine Deposits. Mechanically formed 249
Assortment of Material. Shingle Beaches. Sand Flats
and Banks. Silt and Mud-flats. Silting up of Bays.
Deposition increased by Depression. Deposits in Shallow
Seas. Limits of Terrigenous Deposits. Deposits due to
Action of Ice.
Chapter XV.—Marine Deposits. Calcareenis. . . 273
1. Chemical Precipitates ; Action of Marine Organisms ;
Calcareous Mud. 2. Accumulations of Organic Débris ;
Shell Sands ; Limestone Banks ; Coral-reefs ; Theories
of Coral Growth.
Chapter XVL—Deep Sea and Oceanic Deposits. . 294
Blue Muds. Green Marls and Sands. Pteropod Ooze.
Globigerina Ooze. Red Clay. Radiolarian Ooze. Diatom
Ooze. Rarity of Oceanic Deposits among Stratitied
Rocks.
PART 11.—STRUCTURAL GEOLOGY.
Chapter 1.—Crystals and Rock-forming Minerals . 307
Chemical Elements and Compounds. Crystallography.
Rock-forming Minerals. Pseudomorphs.
Chapter 11.—Classification of Rocks .... 326
Meaning of the word Rock. Lithological Classification.
Tectonic Classification. Genetic Classification. Igneous,
Derivative, and Metamorphic Rocks. Determination of
Rocks.
Chapter 111.—Original or Igneous Rocks . . . 336
Mode of Occurrence. Structure. Chemical Composi¬
tion. Mineral Composition. Specific Gravity. Classi¬
fication. Description of Rock-groups. Pyroclastic Rocks.
Altered Rocks.
Chapter IV.—Derivative or Stratified Rocks . . 355
Origin and Composition. Classification and Descrip¬
tion;—(1) Arenaceous Rocks; (2) Argillaceous Rocks;
(3) Calcareous Rocks ; (4) Ferruginous Rocks ; (5) Car¬
bonaceous Rocks.
Chapter V.—Stratification 372
Formation of Strata. Intervals between Beds. The
Law of Vertical Succession. Inter-bedding. Extension
and Thinning Out. Groups and Episodes. Current-bed-
ding. Current-mark.
INTEODUCTION.
Definitions are neyer easy, and it is especially diffi¬
cult to frame an accurate definition of Geology,
because it is not easy to limit the scope of its application.
Two different definitions of the science may indeed be given ;
one describing it in a broad and comprehensive sense, the
other dealing only with its more special and restricted
meaning.
Eegarded from the wider point of view. Geology is not
so much one separate science as the application of all the
physical sciences to the examination and elucidation of the
structure of the earth. Mr. Jukes has well observed '
"that we might without impropriety regard all the physical
sciences as included under two great heads, namely, astro¬
nomy and geology ; the one comprehending all those sciences
which teach us the constitution, the motions, the relative
places, and the mutual actions of the astra, or heavenly
bodies; while the other would include all the sciences
which deal with the phenomena presented by the one
astrum on which we live, and investigate the nature and
distribution both of the inorganic matter of the globe, and
of the living beings which inhabit it."
The truth of this large and comprehensive view of
geology is shown by the very fact of its late appearance in
the world of science. The crude ideas regarding the struc¬
ture and history of the earth, and the nature of fossils
which were prevalent in the seventeenth and eighteenth
centuries, cannot be dignified by the name of geology.
The Geognosy, taught by Werner in the latter part of the
eighteenth century, was little more than a special branch
^ " Student's Manual of Geology," p. 1.
B
2
INTRODUCTION.
of mineralogy; and Desmarest, though an opponent of
Werner's views regarding the origin of rocks, treated of
geology under the head of physical geography.
It was not, indeed, until much progress had been made
in all the physical sciences that geology could possess any
solid foundation or any real scientific status. It was not
until chemists were aÛe to explain the true nature of the
mineral substances which enter into the composition of
rocks ; until geographers had explored the surface of the
earth, and natural philosophers had described the pheno¬
mena of volcanoes and earthquakes, rivers and glaciers,
tides and currents ; until biologists had described and
classified the greater number of existing animals and plants
—it was not until all these essential data had been collected
and studied that the geologist could realize the processes
and operations by which rocks were formed, or coiild utilize
his discoveries of the fossil organisms which they enclose.
This necessary knowledge was not possessed by anyone
before the beginning of the present century, and, con¬
sequently, before this era there was no true science of
geology. Hutton's knowledge of chemistry and mineralogy
was considerable, and his powers of observation and gene¬
ralization were truly remarkable, but his lack of informa¬
tion in respect to other sciences necessarily caused his
" Theory of the Earth " to be a very imperfect view. It
was not till 1790 that the foundations of geology were
laid, when William Smith began to establish the two great
principles which have caused him to be known as the father
of the science, viz., the superposition of stratified rocks,
and the identification of strata by their fossil contents;
ever since that time materials have been rapidly accumu¬
lating for the construction of a true outline of the history
of the earth, so far as it can be learnt from a study of the
roeks that compose its crust.
It does not follow from what has just been said that
every individual geologist must possess a great and accu¬
rate knowledge of the whole circle of physical sciences ; it
is only necessary that this knowledge should exist, and be
readily accessible, so that the geologist can combine the
results obtained by other sciences with his own special
observations, and apply them to the solution of the many
INTRODUCTION.
3
difficult problems whieb arise in the course of his re¬
searches.
Here the reader may well ask, what are the" special
observations which the geologist makes, and what is it
which he regards as his special business? If the wide
scope already indicated be properly given to geology, and
if it claims assistance from every other science which treats
of anything belonging to the earth ; what is it which the
geologist teaches to the students of these other sciences in
return for the knowledge communicated to him ?
The answer to these questions will show that geology has
a more limited and applied meaning, which may be defined
as follows. Geology in this sense is the application of the
knowledge gained by the study of the existing state of
things to the elucidation of the past history of the earth.
The geologist takes the information gained by others re¬
garding the operations that are now in action on the surface
of the globe, and uses this knowledge to explain the results
of the operations which have taken place in the past. In
this way he is able to explain the manner in which the
various rocks have been formed, and the processes by which
they have acquired their distinctive characters and their
relative positions.
The constitution of the crust of the earth, and the his¬
tory of its formation, are thus the special objects of his
study and research, and, as Mr. Jukes observed, the collec¬
tion and co-ordination of the facts on which this history is
founded is the proper and peculiar business of the geolo¬
gist : " the ditch, the cutting, the quarry, the mine, the
cliff, the gully, the river-bank, and the mountain-side, are
his subjects, which he has to examine and dissect." From
the indications observed in all natural and artificial excava¬
tions, he must make out the internal structure or solid
geometry of every district, and note all the facts which will
enable him to explain the geological history of each natural
province or area, until all the accessible portions of the
earth's crust have been explored and described, and the
" record of the rocks " has been read from beginning to
end.
A beginner would probably suppose that in order to
pursue the task of investigating the solid structure of the
4
INTRODUCTION.
earth, it would only be necessary to become acquainted
with the nature and origin of the rocky matters of which
it is composed, and that there would be no occasion for
studying the forms of animals and plants. The fact is,
however, that an acquaintance with organic remains, as
they occur in a fossil state, is even more necessary for a
geologist than a knowledge of inorganic substances.
The cause of this necessity may be briefly stated as
follows. When portions of the earth's crust came to be
examined, it was found that the various rocks of which it
was composed were arranged in a regular series, the several
rock-groups having been formed one after the other during
successive periods, each of long duration. Now, the mineral
substances produced during any one of this vast succession
of ages do not appear to differ essentially from those formed
under like circumstances at another time. We cannot,
therefore, with any certainty discover the order of time in
which the series of rocks was formed, or their original order
of superposition, from an examination of their mineral
characters only. The animals and plants, however, which
lived at any one period of the earth's history were different
from those which lived at other periods. 'There has been
a constant succession of living beings on the earth, different
races, species, and genera following each other in a regular
order, and when that order has once been ascertained it is
clear that any rock containing fossil remains can be as¬
signed to its proper place in the geological series.
There are few formations that are entirely destitute of
fossils, and in districts where the relative position of the
rock-groups is for any cause obscure, or where their original
order has been disarranged by the subsequent action of
disturbing forces, it is chiefly by aid of their fossil contents
that their true succession and superposition can be satis¬
factorily determined.
Practically it has been found that while an acquaintance
with the characters of some twenty or thirty of the com¬
moner minerals is all that a geologist must necessarily
learn of mineralogy, the number of animals and plants,
with the forms and names of which he will have to make
himself familiar, must be reckoned by hundreds.
From the foregoing remarks it will be seen that before
IXïRODUCTION.
5
we can properly understand the appearances presented by
the rocks which compose the crust of the earth, we must
have some knowledge of the agencies by which rocks are
now being formed, and by which the form of the earth's
surface is being modified. Again, before we can attempt
to construct any history of the formation of the earth's
crust we must be acquainted with the leading facts and
conclusions of Palaeontology, as that branch of the science
is termed which deals with the structure and classification
of former existences.
Descriptions of the various natural operations which are
now in progress upon the earth, and of the changes which
they bring about, were aptly termed by Sir Charles LyeU,
" the Principles of Geology " ; since it is only by a careful
study of the changes which are taking place in the present
that we can hope to interpret the many changes which have
taken place in the past.
A knowledge of these first principles ought, therefore, to
form the groundwork or introduction to the study of those
subjects which are, perhaps, the more special province of
geological science.
It seems, then, the most natural and logical treatment
of the subject in the first place to describe the geological
processes which can be seen at work in various parts of the
world, and to show the student how these operations "will
explain the facts observable in the crust of the earth, and
how they will account for the diversified features which
its surface presents. By these steps we can proceed to
the history of the earth since it became a habitable globe,
as interpreted by means of the observations and deduc¬
tions previously arrived at. This treatise, therefore, will
be di"vided into five parts, which may be designated and
defined as follow.
1. Dynamical Geology.—An account of the actual results
produced by the geological agencies now in operation.
2. Geognosy, or Structural Geology.—Being the study of
rocks and rock-masses, without reference to the geological
time of their production. This branch may be divided
into two sections : a. Petrology—the study of the mineral
components of rocks as visible in detached specimens.
h. Tectonic Geology—the study of rock-masses ; their struc-
6
INTRODUCTION.
tural characters, and mutual relations, as visible in the
earth.
3. Physiography.—An explanation of the origin of hills
and valleys, lakes, escarpments, and mountains.
4. Palœontology.—Under this head will be given an out¬
line of the laws which govern the distribution of life, both
in space and time ; but to deal fully with this part of the
subject a separate volume would be required.
5. Stratigraphy, or Historical Geology.—Being a chrono¬
logical account of the successive groups, or series of strata,
which are found in the earth's crust. This will deal chiefly
with the rocks of the British Islands, and will necessarily
be only a condensed account, but prominence will be given
to those facts which enable us to realize the relative posi¬
tions of land and sea, and the general physical geography
of each geological epoch.
PART L
DYNAMICAL GEOLOGY.
SECTION I. Changes produced by the Influence of
Internal or Subterranean Causes.
a h
X
Fig. 1. Diagram of a Segment of the Terrestrial Globe, to show
its probable internal structure.
CHAPTER I.
THE GENEEAL STKÜCTUEE OF THE EAETH.
HE Shape of the Earth, and its Consequences.
—Everyone knows, or should know, that the earth has
been both measured and weighed ; and that the form of
the globe is not that of à perfect sphere, but of an oblate
spheroid. According to Mr. Clark, the equatorial diameter
of the earth is 7925"604! miles in length, while the length
of the polar diameter is only about 7899T14 miles ; the
difference between them is, therefore, a little less than 26*5
miles. That is to say, that the polar axis is almost exactly
26i miles shorter than an equatorial axis or line passing
through the centre of the earth from one side of the equator
to the other : or, again, as calculated along radii from the
centre, an equatorial radius is about 13^ miles, or 70,000
feet longer than a polar radius.
If, therefore, a true sphere were to be described within
the earth, with a radius equal to that of the polar radius,
the surface of this sphere would lie at the depth of 70,000
feet below the actual surface at the equator.
Now the deepest soundings in the ocean are less than
28,000 feet, and the highest mountain in the world, viz..
Mount Everest in the Himalayas, is only 29,000 feet above
the sea. Compared, therefore, with the bulging protube¬
rance of the earth's mass at the equator, the irregularities
of its surface, which we know as ocean-depths and moun¬
tain heights, are obviously small.
It is also seen that this equatorial bulge of 70,000 feet,
though only about of an equatorial radius, repre¬
sents the bulk or weight of an enormous mass of matter,
and its existence involves very important consequences.
10
dynamical geology.
[sec. i.
The first of these consequences is the stability of the
earth's axis. The earth is a body which revolves upon its
own axis, that is, upon an imaginary line connecting its two
poles, and this line is its shortest diameter. The actual
circumference of the earth's equator is about 83 miles
greater than that of the true sphere enclosed by it, con¬
sequently its linear velocity is more rapid than that of
the inner sphere, because any point on it is carried round
that greater space in the same period of time. Any cause,
therefore, which tended to make the earth revolve upon
any other axis than that of its shortest diameter, would
have to overcome the resistance of the greater momental
force residing in the equatorial protuberance ; and ever
since the earth first acquired its present form, it is very
dif&cult to imagine any cause sufficient to have produced
this result.
Hence we may conclude that the position of the earth's
axis has remained unchanged throughout all geological
time, or at any rate that it has never varied much from its
present position, for there are some facts which are difficult
to explain without imagining a certain amount of change
in the position of the north pole ; they cannot, however, be
taken as proving such a change, and the poles have doubt¬
less always been near their present positions.
The second conclusion deducible from the peculiar shape
of the earth is the fact that it is almost exactly that of a
spheroid of rotation ; in other words, it is the form which
the globe would have assumed had it been originally a
pasty or fluid mass revolving with its present velocity.
The fact that the earth has this form raises a strong
presumption that its condition was at one time sufficiently
plastic to allow it to adjust its shape to the impulse of its
motion ; in short, that it was once in a fluid, semi-fluid, or
pasty condition. Although this original plasticity is by no
means a certainty, yet it is generally assumed as a proba¬
bility, agreeing very well with some other known facts
which are in favour of the idea, that the earth has been
continually parting with heat, and that its own heat was
once sufficient to keep its substance in a molten or semi-
molten condition.
Internal Temperature of the Earth.—Without,
CHAP. I.J GENERAL STRUCTURE OF THE EARTH. 11
then, further considering the question of the possible tem¬
perature and condition of the earth in the earliest times,
we will pass to a subject with which geology is more
directly concerned ; viz., the probable condition and tem¬
perature of its interior at the present time.
This is, of course, a very difiScult matter upon which to
obtain any evidence, but the following facts tend to show
that it consists of a cool envelope surrounding a very hot
interior, and that the latter possesses a high temperature
of its own, quite independently of any heat derived from
the sun or other external source.
1. Volcanoes.—The numerous volcanic orifices, which
occur here and there all over the earth, and from which
streams of lava or molten rock are so often poured forth,
prove that some parts of the interior. at any rate are so
heated that solid rocks are rendered perfectly fluid. But
the lavas derived from existing volcanoes form only a
small portion of the bulk of igneous rocks in the crust
of the earth, all of which have certainly come from some
part of the interior.
2. Temperature of Deep Mines.—The influence of the
summer's heat or winter's cold does not penetrate far into
the interior of the earth, and it is estimated that at a
depth of about fifty feet beneath any given place on the
earth's surface a thermometer would mark the same tem¬
perature all the year round.
Below this limit of seasonal variation, sometimes called
" the stratum of invariable temperature," and as far as
any mines or borings have been carried into the earth, the
temperature of the rocks always increases with the depth.
This downward increment of temperature is not the same
at all places, being sometimes rather more and sometimes
rather less than the average, but there are many causes
which would tend to produce variations in the rate at
which the temperature increases downwards. One of the
most important is that of the relative conductivity of the
rocks, some kinds of rock being much better heat-con¬
ductors than other kinds, so that the rate of increase
varies even in different parts of one boring.
Some of the earliest observations on the internal tem¬
perature of the earth were made by Mr. Henwood in
12
DYNAMICAL GEOLOGY.
[sec. i.
the mines of Cornwall and Devon. The result of his
experiments in two hundred of these mines showed
that the average temperature at different depths was as
follows :—
Depth. Temperature.
At 200 feet .... About 57° Fahr.
„ 600 „ . . . . „ 62° Fahr.
„ 900 „ . . . . „ 68° Fahr.
„ 1,200 „ . . . . „ 78° Fahr.
The average increase below 300 being about 1° Fahr, for
every 43 feet of descent. In the granite rocks, however,
the temperatures were not quite so high.
In a coal-mine at Monkwearmouth, Durham, the tem¬
perature at a depth of 1,600 feet ranges between 78° and
80°, and observations showed that the increase in de¬
scending from the surface was about 1° Fahr, for every
60 feet.
In the Rose Bridge Colliery, near "Wigan, however, a
shaft has been carried down to a still greater depth, viz.,
2,445 feet, and a careful series of observations were made
by the manager, Mr. Bryham, during the process of sink¬
ing. The following are some of the results obtained :—
At a depth of 564 feet, a temperature of 66° F.
1,674 „ „ 78° F.
2,013 „ ,. 86° F.
2,445 „ „ 94° F.
Assuming the surface temperature in this instance to be
50° F., these observations show an average increment of 1°
for every 55 feet of depth.
While, then, there is some variation in the rate of this
increase at different places, it is an undoubted fact that
the temperature of the rocks does always increase after
passing below the first 50 feet ; and it appears that the
average rate of this increase of temperature may be taken
at about 1° F. for every 51 feet of depth.
3. Temperature of the water in deep wells and borings.—
The temperature of the water at the bottom of deep borings
is in agreement with this conclusion.
The water of the artesian well of Grenelle, near Paris,
CHAP. I.J GENERAL STRUCTURE OF THE EARTH. 13
which rises from a depth of 1,800 feet, has a constant
temperature of nearly 82° F., -which is about 31° hotter
than the mean temperature of the ground beneath Paris.
This, therefore, indicates a rate of increase of 1° for every
68 feet of depth.
Another well near Paris, at La Chapelle, where observa¬
tions were taken at different depths, gave the following
results :—■
At 330 feet a temperature of 59|-° F.
» 660 „ „ 62° F.
„ 1,000 „ „ 65T° F.
„ 2,200 „ „ 76° F.
These figures yield a slower rate of increase, viz., only 1° F.
for every 82 feet.
At Sperenberg, near Berlin, a boring was made to a
depth of 4,172 feet, nearly all through rock-salt, and the
temperature observed at a depth of 4,042 feet, when
corrected for pressure, gave a result of 1° F. for every 51u
feet.^
At Kentish Town (London) a well was sunk to a depth
of 1,302 feet, and the temperature at the depth of 1,100
feet was found to be 69'9, giving a mean rate of increase
from the surface of 1° F. for every 54 feet.
4. Hot Springs.—The Geysers and other hot springs
which are found in volcanic districts, must be regarded as
deriving their heat from the volcanic focus, and cannot be
taken as affording independent evidence of the internal
temperature of the earth. Warm springs, however, are
not confined to such districts, but often occur in localities
which are far removed from any recent or recently extinct
volcanoes. In our own country the waters of Bath, with
a temperature of 120° F., and those of Buxton, with a
temperature of 82°, are instances.
There is little doubt that the waters of these springs
travel upwards through cracks and fissures, which originate
at great depths, and that they are hot in consequence of
the depth from which they come.
^ For a full account of the temperatures in this horing, and their
teaching, see Fisher's "Physics of the Earth's Crust," 2nd ed.,
p. 7.
14
dynamical geology.
[sec. i.
Internal Condition of the Earth.—The combined
evidence of all the preceding considerations leads us to con¬
clude that the interior of the earth is intensely hot.
Assuming the temperature of the invariable stratum
beneath the British Isles to be 50° F. ; and that below this
the increase of temperature continues indefinitely into the
interior at the same rate as that which obtains in our mines
and wells, viz., about 1° for every 51 feet or 103^° for every
mile, we should shortly arrive at a very high tem¬
perature.
At a depth of 8,260 feet or less than two miles, the rocks
would be as hot as boiling water, or 212° F. ; at a depth of
27 miles they would be at a temperature of 2,794°, which is
a heat sufScient to melt steel ; and at 40 miles, a depth
which is really only a little way towards the centre of the
earth, we should find a temperature of 4,140°, which is
a greater heat than any that we can produce at the
siu-face.
It does not follow, however, as a necessary consequence
that the materials which exist at these great depths would
be melted at the same temperature that would fuse them
at the surface, since the enormous pressure which they
must sustain may keep them solid in spite of the heat.
It is well known that pressure retards or raises the
melting-point of most solids, that is to say, an increase of
pressure increases the temperature at which each is fusible.
Very little, however, is known about the exact relations
between pressure and the fusing-point of rocks, and it is
possible that different substances are affected by pressure
in a different ratio.
It has been supposed that the ratios of pressure and
temperature within the earth are such that, below a certain
depth, the pressure is only just sufficient to prevent the
liquefaction of the rocks by the temperature proper to that
depth, so that any local cause diminishing the pressure
would allow of liquefaction.
Again, it is possible that the ratios of the increase of
pressure and temperature are such that, at a certain depth,
the pressure is insufficient to prevent liquefaction, and,
consequently, that a certain thickness of the earth's mass
is liquid ; but that, at a still greater depth, the increased
CHAP. I.] GENERAL STRUCTURE OF THE EARTH.
15
pressure overcomes the influence of heat, and keeps the
rocks solid, in spite of increased temperature.
Whether solid or fluid, we may safely assume that the
interior of the earth has a very high temperature of its
own, and if this is so, it must have been continually losing
heat by conduction or convection from the interior to the
surface, and this heat must have been dissipated into
space. Consequently we are led back to the conclusion,
which was foreshadowed on p. 10, that there was a time
when the whole mass of the earth was in a molten or in¬
candescent state, and that it has gradually cooled down to
its present condition.
Assuming this to be the case, the condition of the earth's
interior at the present time must depend upon the manner
in which it has cooled, and Mr. Hopkins has shown that
the earth's mass must now be in one of three different
states ; these are,'
1. A solid crust with a fluid interior.
2. A solid crust and a solid nucleus with a fluid inter-
stratum.
3. Solid throughout, with or without fluid spaces or
cavities.
Hopkins concluded that the balance of probability was
in favour of the last (No. 3). He admitted,^ however, that
after the solidification of the nucleus there must have been
a time when a solid external crust was formed resting on
an imperfectly fluid mass beneath ; but he thought that it
must have grown to a thickness of not less than 800 or
1,000 miles at the present time, and that it had probably
united itself to the internal nucleus.
He bases his opinion on the ground that the movements
of precession and nutation could not be as they are, unless
the crust possessed such a minimum thickness. Sir W.
Thomson, however, has recently shown that these move¬
ments would still be experienced in the case of a thin crust
covering a fluid substratum if the inner surface of the crust
in contact with the fluid was not perfectly spherical. If
this surface was only slightly irregular, the fluid and the
' Phil. Trans. Koy. Soc., 1839.
" Brit. Assoc. Rep., p. 48, note.
16
DYNAMICAL GEOLOGY.
[sec. I.
crust " would have sensibly the same precessional motion
as if the whole constituted one rigid body." '
Sir W. Thomson has nevertheless expressed his con¬
currence with Mr. Hopkins' conclusion that the earth is
either solid or possesses so thick a crust as to give it prac¬
tical solidity. He considers that the tide-producing power
of the sun and moon is so great that if the thickness of the
crust was less than 2,000 miles, it would yield to the strain,
and would be subject to periodic deformations similar to
oceanic tides." He says that, " the solid crust would yield
so freely to the deforming influence of the sun and moon,
that it would simply carry the waters of the ocean up and
down with it, and there would be no sensible tidal rise and
fall of water relatively to land." He concludes that the
mass of the earth "is on the whole more rigid than a con¬
tinuous solid globe of glass of the same diameter."
Prof. Gr. H. Darwin has also investigated the tidal defor¬
mation of viscous spheroids, and flnds that Sir W. Thom¬
son's results must be modified. In his first investigation
(1879) he came to the conclusion that the effective rigidity
of the earth was great, but not so great as that of glass or
steel ; and he adds that " no very considerable portion of
the interior of the earth can even distantly approach the
fluid condition." "
Still more recently Prof. Darwin has returned to the
problem, which is one of extreme complexity, and obtains
a result which he expresses as follows : " The present re¬
sult shows that it is not possible to attain any estimate of
the earth's rigidity in this way ; but as the tides of the
long period are distinctly sensible, we may accept the inves¬
tigation in the " Natural Philosophy " as generally confirma¬
tory of Thomson's view as to the effective rigidity of the
earth's whole mass." "
Lastly, Mr. Fisher points out ' that the most recent obser¬
vations throw some doubt on the actual existence of the
' Brit. Assoc. Kep., 1876. Sectional Address, p. 5.
' Loc. cit., p. 7.
' Phil. Trans., 1879, Part I.
* " Nature," Jan. 20th, 1887.
' " Physics of the Earth's Crust," 2nd ed., p. 41, and Appendix,
p. 34.
CHAP. I.J GENERAL STRUCTURE OF THE EARTH. 17
small fortnightly tide, on the theoretical existence of which
the calculations of Thomson and Darwin are based. The
whole matter is, in fact, fraught with so many difficulties
and uncertainties, that geologists need not consider the
results obtained as in any degree approaching to mathe¬
matical demonstrations.
Mr. Fisher, moreover, meets the whole case of the astro¬
nomers by the following hypothesis : he assumes that there
is a liquid substratum, suggesting that it consists not of
melted roch only, but of an intimate mixture of fused roch
and gas, the gas being, in fact, dissolved in the liquid
magma. This gas may be pure hydrogen, or it may be
some compound of hydrogen, such as water, maintained in
the state of a gas by the great heat of the interior.
This supposition of a liquid substratum containing dis¬
solved gases is supported by much geological evidence.
Steam and gases of various kinds are given off in large
quantities during volcanic eruptions. The observations of
Siemens on Vesuvius in 1878, and of Fouqué on the erup¬
tions of Santorin (1810), have shown that hydrogen is one
of the most abundant gases in such eruptions. Siemens
concluded that vast quantities of hydrogen gas, or of com¬
bustible compoimds of hydrogen, must exist in the earth's
interior, and that portions escape through the volcanic out¬
lets. It is also supposed to be the rise of these super¬
heated gases which keeps the lava inside a volcano hot and
liquid.
Mr. Fisher therefore assumes the existence of a liquid
substratum which is saturated with gases, and is there¬
fore expansible ; and he endeavours to ascertain how such
a magma would behave in respect to the formation of tides.
The materials for a proper examination of this problem do
not yet exist—one of the desiderata being experiments on
the capacity of molten rock (such as slag) for dissolving
gases ; but if the case is similar to the solubility of certain
gases in water, and with certain other conditions which are
not improbable, Mr. Fisher shows that there would not
necessarily be any tide in the substratum which would
affect the level of its surface, or cause any rise or fall of
the overlying crust.
Mr. Fisher's argument if sound is of great importance to
c
18
DYNAMICAL GEOLOGY.
[sec. i.
geologists, who have always felt great difficulty in account¬
ing for various geological phenomena on the supposition of
the earth being a solid and rigid globe ; they have always
been inclined to believe in the liquidity of some portion of
the earth's interior, for it is only on this assumption that
they are able to explain the occurrence of volcanoes, earth¬
quakes, and other earth movements, or the fact that vol¬
canic products are similar all over the world, as if derived
from a common source.'
Assuming the existence of a continuous liquid substra¬
tum, Mr. Fisher adduces physical arguments for supposing
that the thickness of the overlying crust is not everywhere
the same; that its average thickness at the sea-level is
about 25 miles; that it is thicker than this tmder mountain
chains, and thinner under the oceans. To this question we
shall have occasion to recur in the sequel.
In this chapter we have briefly discussed the views
which have been held by the best modem authorities re¬
specting the structure and condition of the earth's interior.
We have seen that the most probable conclusions are—
1. That the interior is very hot.
2. That a portion of it is Hquid, and forms a continuous
layer between the solid crast and the central mass, whether
that be solid or plastic (see flg. I, p. 8).
3. That the solid crust is comparatively thin.
4. That the liquid layer is saturated with dissolved
gases.
It is obvious that under such conditions the earth's crust
is not likely to be perfectly rigid and unyielding, and that
if any causes exist which tend to alter the equilibrium of
the cmst, movements of various kinds are likely to be
produced. The crust, in fact, would give evidence of
decided instability.
As a matter of fact we know that such movements do
take place all over the globe. Volcanoes pour forth vast
quantities of molten matter ; earthquakes shake large
^ Another mathematician. Professor Harkness, has also admitted
the force of the arguments against the solidity of the earth, and
the possibility of a thin crust resting in hydrostatic equilihrium on
a denser sulistratum. (" On the Solar Parallax," Washington,
1891.)
•CHAP. I.J GENERAL STRUCTURE OF THE EARTH. 19
tracts of ground, and often produce permanent changes in
the relative level of land and sea; and lastly, there is
evidence to prove that slow and gradual movements of the
crust have been in progress over still larger areas, the land
being elevated in one place and depressed in another.
Finally it will appear that every part of the land has been
below the sea at some period of past time, and is now dry
land only because it has been elevated by the movements
above mentioned.
It is important that the geological student should thus
early realize the fact of the instability of the earth's crust,
proofs of which will be given in the next four chapters, for
he will find evidence of the strains, pressures, and move¬
ments to which the rocks have been subjected in every
branch of geological study.
CHAPTEE II.
VOLCANOES.
General Form and Structure.—A volcano may
be defined as a conical pile of materials which have
been ejected from the orifice of a pipe or fissure communi¬
cating with a highly heated portion of the earth's crust
below. The materials composing the cone are chiefiy frag¬
ments of rock which have been violently discharged into the
air, and have fallen on all sides round the central orifice or
crater as it is called. Every eruption adds to the pile and
increases the height and magnitude of the volcano ; great
blocks as well as small stones, cinders, and ashes are thrown
up and fall on all sides of the mountain. Floods of lava
now and then boil up, and either break through the sides
of the cone or run over the lip of the crater, and fiow
down into the adjacent country in the form of lava streams.
Successive eruptions frequently take place from the same
vent, but minor cones are often formed on the flanks of the
larger one, and lesser eruptions proceed from them.
Volcanoes are sometimes isolated mountains like Vesuvius
and Etna, but are more frequently arranged as a row of
cones and peaks along straight or slightly curved lines, as
if they had been thrown up at intervals along the course of
great linear fissures in the earth's crust ; such are the
chains of the Andes, the Aleutian Islands, and the Malay
Archipelago. These lines of volcanic vents sometimes seem
to radiate from a special volcanic centre.
If we could make a vertical cut or section through a vol¬
canic hill, so as to lay open its internal structure from the
summit to the ground it rests upon, we should find this
structure to be similar to that suggested in the diagram.
CHAP. II.]
VOLCANOES.
21
fig. 2. At the base lies the platform of stratified rocks
upon which the volcanic cone is reared. The parts shaded
by vertical lines are intended to represent the lava-streams
which have at different times issued from the crater and
roUed down the surface slopes; the intervening parts,
shaded with fine lines and dots, indicate the successive
layers of stones and ashes which have been ejected from
the central vent, and are disposed in more or less regular
layers inclined at various angles, but all sloping away from
the central axis. On either side are seen minor cones com¬
posed entirely of dust, cinders, and stones which have been
thrown up from these lateral vents. Lastly, the lava which
has risen from below is represented by the thick black
lines which intersect the mountain in aU directions.
Fig. 2. Ideal Section through a Volcanic Cone.
It will readily be imagined that when molten lava is being
forced up inside a volcano, some of it will be injected into
any cracks or fissures which may exist, or which are caused
by the impelling force. Sometimes the lava cools and
soHdifies in these cracks without making its way to the
surface, forming veins or walls of solid stone which are
called àykts by geologists. Such dykes occur by hundreds
in every volcanic cone traversing the previously formed
layers of ash and lava, and sending out smaller veins in all
. directions. Fig. 2 is of course merely a diagram, and the
internal structure of a volcano is in reality much more
complicated than is there represented.
The above is the structure of an ordinary volcanic moun¬
tain, but there is another form of volcano, less commonly
22
dtnamicai; geology.
[sec. i.
found, which consists almost entirely of lava-flows. Such
a volcano does not form a conical mountain, but a broad
low dome-shaped one, its slopes being seldom steeper than
10°, whereas those of ordinary volcanoes often slope at
angles of 28° and 30°. Volcanoes of this type do not
possess a true crater, but only a central pit or caldera, and
though the lava sometimes issues from this, it more often
breaks forth from the flanks of the dome. The lavas
ejected from these openings sometimes accumulate till they
form a huge mound-shaped mass many thousand feet high,
and covering an immense area of ground. Fig. 3 repre¬
sents the outline of such a mound, in which the solidified
lava-flows would show in section as so many overlapping
lenticular cakes intersected by dykes of lava leading up to
the orifices from which the lavas have issued.
Phenomena of an Eruption.—The two types of
volcanic vents above described correspond with certain
special characteristics of the eruptions by which they have
been formed : the one kind may be termed explosive erup¬
tions, and the other kind non-explosive eruptions.
a. Explosive Eruptions.—These usually exhibit the fol¬
lowing succession of phenomena. The first intimations of
the disturbance are generally loud rumbling sounds which
seem to proceed from the interior of the mountain. Shocks
of earthquake frequently succeed these noises ; the water in
wells and springs sink down in consequence of the opening
of rents and fissures below ; and finally, owing to the trans¬
mission of submarine shocks, the sea often ebbs and flows
in an unnatural manner.
After these general disturbances, which are suficiently
alarming, the phenomena begin to be concentrated in thei
crater, and usually occTir in the following order :—
1. An explosive discharge of steam, falling in rain.
2. A discharge of dust, ashes, and stones.
3. The ejection of larger blocks with heavier explosions :
CHAP. II.J
VOLCANOES.
23
at the same time the red glow of the lava rising in the
funnel lights up the vapour-clouds above, and produces the
appearance of flames.
4. The lava rises and flnds egress either from the crater
or from lateral openings.
5. "When the flow of lava ceases there is frequently a
discharge of black sand or comminuted pumice. In some
cases the eruption does not proceed farther than stage
No. 2 ; but in violent eruptions there is a continued repeti¬
tion of heavy explosions and outflows of lava.
Some volcanoes are in a state of constant activity, small
explosions taking place incessantly at short intervals, while
occasionally more violent outbursts occur, and streams of
incandescent lava are poured forth. In other cases periods
of quiescence alternate with periods of activity, and
when eruptions do occur they are generally of a violent
description.
From the records which we possess relating to the Italian
volcanoes, it would appear that the same volcanic centre
may exhibit both phases at different times ; the eruptions
being sometimes incessant and of moderate intensity for a
long time, then ceasing, and a state of quiescence ensuing,
which is broken by a more violent oufburst. It has been
noticed, indeed, that there is a distinct relation between the
violence of an eruption and the length of the intervening
periods of repose, so that the following rules may be laid
down : '—
1. Feeble and short eruptions usually recur at brief
intervals.
2. Yiolent or long-continued eruptions are generally
followed by long periods of repose.
8. A long period of repose is often succeeded by an erup¬
tion which is either of long duration or of great violence.
Truncation of the Cone.—Sometimes a violent outburst
of the volcanic force will blow the upper part of the cone
completely away, and thus destroy what it has taken ages
to build up. The volcano then presents the appearance of
a truncated cone with a huge gaping hollow in the centre.
Subsequent eruptions, however, repair the damage by
' See ""Volcanoes," by J. W. Judd, p. 33.
24
DYNAMICAL GEOLOGY.
[sec. i.
forming a new cone in the interior of the great hollow ; so
that the mountain eventually consists of a lofty circular
ridge surrounding a central depression, from the floor of
which rises the new and active volcanic cone (as indicated
in fig. 4). This formation of a new cone within an older
truncated one is of frequent occurrence in the history
of volcanoes. Vesuvius is known to have presented this
appearance at several periods, and is even now partially
surrounded by the old crater-ring called Monte Somma
(see fig. 5).
Barren Island, in the Bay of Bengal, is an excellent
instance of this form of volcano, as represented in fig. 4.
Here the whole central part of the mountain has been
blown out by one tremendous explosion, the outline which
Fig. 4. Outline of Barren Island, after Scrope.
the cone must have originally possessed being indicated by
the dotted line.
The Peak of Teneriffe is another example in which the
central cone rises far above the encircling ring of the older
truncated mountain.
b. Non-Explosive Eruptions.—These are not generally
preceded by any violent disturbances, though occasionally
earthquakes of some severity occur. As a rule also there
are no explosive discharges of steam, dust, or stones, but
sometimes small discharges of this kind are thrown up
from points along the fissures which are rent through the
mountain. The rise of the lava in the central pipe of the
volcano is generally slow, sometimes rising and falling
again, till it either flows over the lip of the caldera or bursts
a passage at some point lower down. When the latter is
the case, it is forced up in.jets or fountains of red-hot liquid,
from which rivers of lava course down the slopes ; these
CHAP. II,]
VOLCANOES.
25
lavas being generally so fluid that they form streams 50
or 60 miles in length. Our knowledge of these kind of
eruptions is chiefly derived from the volcanoes of Hawaii in
the Paciflc Ocean.
Products of an Eruption.—These may be classified
and described in the following order ;—1. Steam and gases ;
2. Stones and ashes ; 3. Lava.
1. Steam and Gases.—During an explosive eruption
steam is always discharged in great quantities, bursting
forth with a roaring sound from the crater, and sometimes
also from vents and fissures on the sides of the mountain.
The perpendicular uprush of steam from the crater spreads
out above into a great horizontal cloud of vapour, the out¬
line of the column and cloud together seen from a distance
bearing much resemblance to that of a pine-tree very
common in Italy, and known as the stone-pine (see fig. 5).
Around this column vivid flashes of lightning are
constantly playing, the electricity being produced by the
friction of the uprushing current of steam as it escapes
through the shaft or funnel of the volcano, and partly
also by the collision of the fragments which are carried
upward with it. The air at length becomes loaded with
vapour which falls in heavy rain, thunder-storms are fre¬
quent, and great floods are thereby occasioned. The rain¬
water courses down the slopes and sweeps along the loose
volcanic dust and ash, forming muddy torrents, which often
work more damage and destruction than the streams of
lava.
-Besides steam the following gases are emitted from
volcanoes :—carbonic acid, hydrochloric acid, sulphurous
acid, sulphuretted hydrogen, and boracic acid ; together
with hydrogen, nitrogen, and ammonia. By the action of
these gases on the materials of the cooling lavas and
surrounding rocks, a number of secondary products are
formed, consisting of the chlorides and sulphates of various
earths, sulphur and sulphides of certain metals. When
sulphurous acid and sulphuretted hydrogen come into con¬
tact with each other, a chemical action takes place, which
results in the formation of water and sulphuric acid, and
the liberation of a certain amount of pure sulphur. This
sulphur remains in the fissures and cavities where the com-
26
dynamical geology.
[sec. r.
bination takes place, the water is evaporated, and the sul¬
phuric acid forms sulphates by combiuiug with lime, iron,
soda, and other substances.
2. Fragmental Products.—The solid materials ejected
during an eruption are of various sizes and kinds. Some
are fragments of the rocks which underlie the volcano,
such as limestone or shale. These have been broken off,
and blown out by the force of the explosions ; but the
greater number of the ejected fragments are pieces of molten
rock or lava. The viscid scum which forms on the sur¬
face of the rising lava prevents the free escape of the steam
which is constantly ascending through the mass ; the steam,
therefore, collects into great bubbles, which eventually
burst, and the pressure being then relieved, a general
escape of steam takes place with explosive violence, and
fragments of the semi-liquid material are hurled high into
the air. Among these there are sometimes blocks of large
size, which, rotating as they rise, come to assume a
spheroidal form ; these are called volcanic bombs. Smaller
cindery fragments are termed cinders or scorice. Some of
these blocks and cinders fall outside the crater ; but many
fall inside, and are ejected again and again, hurtling against
one another in the air, till they are reduced to fragments of
still smaller dimensions (lapilli), while great quantities of
mere dust and ash are produced at the same time.
The accumulations of coarser déèm include fragments of
all sizes, and form the rock known as volcanic agglomerate.
In some cases blocks derived from the strata underlying the
volcano are very numerous, but according to Mr. Johnston-
Lavis they are only ejected during very violent explosive
eruptions, when the pit of the crater extends down into the
foundation rocks. " Another group of ejected blocks con¬
sists of coarsely crystalline varieties of the lava that usually
issues from the volcano, and these are no doubt portions of
the magma that have cooled as dykes during a period of
quiescence, and have subsequently been torn off by ex¬
plosions." ^ The stones and cinders which fall on the in¬
ward slopes of the crater roll down again into its mouth ;
' Johnston-Lavis on " Ejectamenta of Volcanoes," Proc. Geol.
Assoc., vol. ix. p. 432.
Fig. 5. Vesuvius as it appeared during the eruption of Oct., 1822.
28
dynamical geology.
[sec. I.
and, at the close of an eruption, the crater and funnel
are sometimes choked up with an agglomerate of these
materials.
The rain, which is constantly falling during a long-con¬
tinued eruption, mingles with the fine volcanic dust, and
produces a peculiar pasty kind of mud, which sets into a
firm, but light and porous stone, and is known by the name
of tuff or tufa. It often encloses and is partly made up of
lapilli, scoriae, and broken crystals, and is then generally
known as volcanic ash, or by the French cinerite, and by
the Italians peperino.
The more liquid and glassy lavas, when distended by the
formation of steam, give rise to a kind of lava-foam or froth,
which is often ejected in large quantities, and when cooled
forms the porous grey or whitish substance known as
pumice. " What really occurs," says Mr. Johnston-Lavis,
"is an intermolecular separation of the dissolved water, its
conversion into steam, and its union in bubbles of all sizes,
which, owing to the rapid cooling, are prevented from
escaping except on the surface, as is shown by the more
compact crust of pumice-masses." (Op. cit. p. 424.)
A strong wind blowing at the time sometimes causes a
greater accumulation on one side of the mountain than on
the other. The finer dust and powder is often ejected to
such a height as to be carried by the upper currents of the
atmosphere for hundreds and even thousands of miles
before being dropped. In 1835 the dust and ashes from
Coseguina, in Nicaragua, fell in the streets of Kingston,
Jamaica, 700 miles distant, while the groimd for 80 miles
south of the volcano was covered with a layer ten feet
thick, destroying the woods and dwellings, and enveloping
thousands of animals which were rmable to escape from the
sudden fall of the dense dust-cloud.
During the eruptions of 1874-75 in Iceland, large tracts
of the adjoining country were covered with a thick blanket
of ashes, and fine dust feU in considerable quantities over
certain parts of Norway and Sweden.
The great eruption of Krakatau, between Java and
Sumatra, in 1883, is memorable not only for its terrific
violence and destructive force, but for the prodigious
quantities of dust and pumice which were ejected ; the dust
CHAP. II.]
VOLCANOES.
29
was carried into the higher regions of the air and travelled
to immense distances, producing atmospheric phenomena
over the greater part of the globe ; the amount of pumice
was so great in the Straits of Sunda that vessels had some
difficulty in forcing their way through it, and thence, floating
on the water, it was driven by the winds and currents over
the greater part of the Indian Ocean.
3. Lavas.—Lava-streams vary considerably in their de¬
gree of fluidity, and, consequently, in the rapidity with
which they flow. In some cases the lava is so liquid that
it pours rapidly down the slopes of the cone like a river,
and flows far out from the base of the volcano ; but more
usually it only preserves its fluidity for a short distance
from the point of exit, a rough slaggy crust quickly form¬
ing over the surface of the coulée, though the lava beneath
is still viscid, aud the whole mass continues to move
forward.
A viscous lava which is saturated with gas always has a
rough, cindery, and vesicular crust, so much so that its sur¬
face is sometimes broken up by the rapid movement into
large cindery blocks with sharp ragged projections. The
lower end of such a stream has been described as a slowly-
moving mass of loose porous blocks, gradually rolling and
tumbling over each other, with a loud rattling noise, as they
are forced along by the pressure of the viscid mass of cool¬
ing lava below. A more liquid lava, containing less gaseous
matter, generally has a crust that is wrinkled and twisted
into ropy forms, such as are often seen on the surfaces of
artificial slags.
The liquidity of a lava seems to depend partly on
chemical composition, partly on the amount of gaseous
matter which it includes, and partly on the temperature it
possesses at the time of ejection. As regards chemical
composition, lavas differ principally in the amount of silica
they contain, which varies from about 45 to 80 per cent.
Those with a high proportion of silica are known as acid
lavas, and when cooled they form Ehyolite and Obsidian ;
those with only 45 to 55 per cent, of silica are termed basic
lavas, and when cooled they form Basalt and its varieties.
Between these two extremes there are various intermediate
kinds, such as form Trachyte and Andésite.
30
DYNAMICAL GEOLOGY.
[sec. i.
The amount of gas and steam given off during an erup¬
tion, and from the surface of lava-flows, varies considerably.
Many lavas, such as those of Vesuvius, are so saturated
with water-gas that the flowing lava is often quite concealed
from view in a dense cloud of steam, which is blown off from
hundreds of small vents and spiracles all over the surface
of the stream. Other lavas, both acid and basic, well up
more quietly, and contain but little steam or gas, or else
they part with it in a less explosive manner.
Lastly, as to the temperature of lavas at the moment of
issue, there is little doubt that this differs in different cases
and places, but its actual amount in any case has not yet
been ascertained, owing to the difficulties of observation.
Copper wire has been melted in lava which indicates a heat
of about 2,200° F. The complete fusion of the basic slags
which are formed in an iron furnace occurs at about
2,700° F., and we may therefore assume that the tempera¬
ture of liquid basic lava is at least as much, while for the
fusion of an acid lava a greater heat must be necessary.
Lavas, however, are not always in a state of complete
fusion ; observations have shown that it often consists of a
pasty or semi-fluid mass, in which are floating a number of
crystals and crystalline particles of several different
minerals. These crystals bear evidence to having been
formed under great pressure, and in the presence of super¬
heated steam or gas, and have evidently originated while
the lava was in the pipe of the volcano. As the lava cools
the pasty mass also crystallizes, and forms a finely crystalline
stone enveloping the larger separate and previously-formed
crystals.
As all rock is a bad conductor of heat, so, when the crust
has once formed, the internal portion of the lava-stream
always remains hot for a long period of time. The process
of solidification is, therefore, a very gradual one ; and a
great thickness of lava takes a proportionately long time to
cool, and solidify into stone. Natural sections of lava-
streams in cliffs or ravines show that the upper and lower
layers of the cooled mass are always more or less vesicular
and scoriaceous, while the central portion is a hard, com¬
pact, and frequently a crystalline stone.
The gradual cooling of the lava causes it to contract, and
CHAP. II.J
VOLCANOES.
31
leads to the production of cracks, which traverse the whole
mass from top to bottom. These are sometimes so regular
as to split the mass up into a series of hexagonal prisms or
rude columns, similar to those assumed by starch and other
substances in passing slowly from a fluid to a solid state.
This structure will be more particularly described in
Part II.
Slow-moving lavas are frequently congealed before they
arrive at the base of the cone, but the more liquid lavas
often flow to a distance of many miles from the point of
eruption. Some lava-currents from~ Vesuvius are said to
have flowed a mile and a half in fourteen minutes, and
others a distance of nearly two miles in the space of three
hours. The stream which issued from one of the lateral
cones of Etna, and destroyed Catania in 1669, was 14 miles
in length, with a width of 5 miles in some places. The floods
of lava, however, which issued from Skaptar Yokul in
Iceland, in the year 1788, are the most extraordinary on
record. One of these lava-streams was 50 miles long, with
a width of from 12 to 16 miles ; another was 40 miles long,
with an occasional width of 7 miles ; both had an average
depth of 100 feet, increasing here and there in narrow
ravines to a depth of 500 or 600 feet.
Formation of Lava-filled Fissures.—The position
which volcanoes apparently hold in relation to lines of
Assure in the earth's crust has already been mentioned ;
and the dykes which traverse the cone itself have been
spoken of as cracks injected with lava. The actual forma¬
tion of such fissures has, indeed, been observed ; and Sir
■Charles Lyell gives the following account of one which
opened in the plain of St. Lio, near Etna : ^—" A fissure
6 feet broad, and of unknown depth, opened with a loud
crash, and ran in a tortuous course to within a mile of the
summit of Etna. Its direction was from north to south,
.and its length 12 miles. It emitted a most vivid light, in¬
dicating that the fissure was filled with incandescent lava,
probably to the height of an orifice not far from Monte
Eossi, which at that time opened and poured out a lava-
current. When the melted matter in such a rent has
' " Principles of Geology," tenth edition, vol. ii. p. 21.
32
DYNAMICAL GEOLOGY.
[sec. i.
cooled, it must become a solid wall or dyke, intersecting
the older rocks of which the mountain is composed." There
is every reason to believe that the fissures over which vol¬
canoes stand are similarly filled with molten rock, and are,
in fact, the feeders or reservoirs of the volcanic vents. On
the cessation of volcanic activity in the district these lava-
filled fissures must remain as enormous dykes, extending
for long distances, and cutting through all the rocks of
which the country is composed. Such dykes are sometimes
found extending for distances of 50 or 100 miles across the
country ; and in Iceland, during the eruption of Skaptar
Tokul, in 1783, " lava was emitted consecutively at several"
points on a linear range of 200 miles," a lava-filled fissure
doubtless extending throughout the whole of this distanöe,
and now remaining as a solid dyke.^
Such dykes, as Lyell observes, may be called the roots of
the volcano, reaching downwards to the regions of subter¬
ranean fire, while the external cone, with its lava-streams
and loose ashes, may be likened to the branches and light
foliage ; and, to complete the simile, we may consider the
central pipe or chimney to be the stock or trunk of the
volcanic tree through which the lava-sap rises from the
deep-seated roots to feed the sub-aerial branches.
Exciting Causes of Volcanic Action.—This is rather
an abstruse subject upon which to enter near the begin¬
ning of a treatise on Geology, but we shall endeavour te
put the results of recent researches as simply and briefly
as possible. With respect to the immediate cause of erup¬
tion—that is to say, the force which causes the explosions,
and raises the lava in the volcano—there is a general agree¬
ment of opinion, and this has been so well expressed by
Professor Judd that his words are here quoted : ^ " That the
molten materials which issue from volcanic vents have
absorbed enormous quantities of steam and other gases we
have the most indisputable evidence. The volume of such
gases given off during volcanic outbursts, and while the
lava streams are flowing and consolidating, is enormous.
... It is to the violent escape of these gases from the
' Scrope, " Volcanoes," p. 52.
^ "Volcanoes," Internat. Scientific Series, second edition, p. 357-
CHAP. II.]
VOLCANOES.
33
molten rock-masses, as tlie pressure upon tkem is relieved,
that nearly all the active phenomena of volcanoes must be
referred ; and it was the recognition of this fact by Spal-
kanzani, while he was watching the phenomena displayed
in the crater of Stromboli, which laid the foundations of
the science of Vulcanology."
But other questions here present themselves : at what
point and in what manner did the gaseous material gain
access to the molten rock, and how did it come to be
occluded or confined within it ? It has been supposed that
the water might be supplied by a downward percolation
from the oceans and other surface waters, and that even at
great depths a capillary conduction of water would still
take place toward the earth's interior ; but although there
is much evidence to show that surface waters do penetrate
to great depths, and that they do actually gain access to
the ducts of volcanoes when once these have been esta¬
blished, yet it is impossible to conceive that water could
penetrate the solid crust of the earth to a depth of 25 or
80 mües, so as to become the initial cause of volcanic
action. Not the minutest crack or interstitial space can
exist at a depth where the rocks must be in a plastic con¬
dition under the infiuence of enormous heat and pressure.
If we could imagine the existence of cavities at such
depths, capillary action might be set up ; but this seems
to be impossible.'
It is much more likely that the water-substance is dis¬
solved or occluded in a liquid substratum of fused rock,
its presence there being accounted for in the same way as
the presence of other oxygen compounds, that is to say,
its materials existed originally in the form of oxygen and
hydrogen gases, and it is only when and where conditions
allow of their combination that water-substance is formed.
If the conditions of the liquid substratum have allowed of
this combination, the water is still a gas dissolved in the
liquid, for it is a well-known fact that all liquids can absorb
large quantities of gaseous matter, and that the quantity
so absorbed (though not the volume) is increased by pres-
' For a refutation of the theory of capillary penetration and vol¬
canic action suggested by M. Daubrée, see Fisher's " Physics of
the Earth's Crust," second edition, p. 143.
D
34
dynamical geology.
[sec. i.
sure. On the relief of pressure, the expansion of the
water-gas would lessen the specific gravity of the liquid
and enable it to rise, while if it reached the surface ex¬
plosive ebullition might take place.
That the escape of such occluded gas can produce vol¬
canic action we have an interesting demonstration in the
phenomena exhibited by cooling sulphur during the process
of its extraction from soda-residues. The molten sulphur
is exposed to a temperature of 262° F. and a pressure of
two or three atmospheres, in the presence of steam. Under
these circumstances it is found that the sulphur absorbs a
considerable amount of steam ; and when the mass is
allowed to cool and solidify, this is expelled again with
great violence. The hardened surface crust is agitated and
fissured, miniature cones and lava-streams being formed
upon it which have a striking resemblance to the grander
phenomena of the same kind which occur on the surface of
the earth.
In the presence of confined steam or gas we have, there¬
fore, a competent cause of volcanic action, if a way was once
made for its escape through the crust of the earth. This
is probably accomplished by the strains to which the lower
part of the earth's crust is subjected, aided by movements
in the form of convection currents occurring in the sub¬
jacent liquid stratum, and resulting in the production of
fissures along certain lines of weakness in the crust ; but
further consideration of the subject must be deferred till
the student has acquired more knowledge of structural
geology ; and, after aU, we can but guess at the exact
modus operandi of the forces concerned in producing a
volcano.
CHAPTEE III.
VOLCANOES (continued).
Examples of volcanic Action.—Vesuvius and
the Phlegrœan Fields.^—The volcanic region of Naples
consists of a linear group of cones, ranging N.E. and S.W.,
and terminated at its extremities by the two principal
mountains, Ischia and Vesuvius ; the latter seems to be
connected by the intervention of minor vents with the
group of Albano and of Eome—the seven hills of the
Eternal City being for the most part volcanic mounds.^
The celebrated mountain of Vesuvius consists of a cen¬
tral peak, half encircled by the remnant of a more ancient
cone, which now forms the ridge known as Monte Somma,
the depression between them being called the Atrio del
Cavallo. Viewed from the south-west its general outline
is exceedingly regular, though its summit is now broken
and truncated. Mr. Scrope observes that this regularity
is owing to the great fluidity of its basaltic lavas, which,
issuing from the central vent, have taken their course in
spreading sheets down the outer slope of the mountain,
while the scoriae and fragmentary substances projected at
the same time into the air were spread pretty evenly over
them.
" The result of successive eruptions of this kind has been
the formation of a regularly conical mountain, with a
gradually diminishing slope on all sides, from the central
heights to the plain around, exhibiting in the ravines that
^ Some of the following passages are quoted from Mantell's
■"Wonders of Geology," seventh edition, p. 843 et seq., and were
chiefly compiled from the writings of Mr. Poulett Scrope.
' See Murchison, "Quart. Journ. Geol. Soc.," vol. vi. p. 281.
36
DYNAMICAL GEOLOGY,
[sec. i.
furrow its sides, as well as in the abrupt sections afforded
by the walls of the great crater, its composition of re¬
peatedly alternating beds of lava, pumice, and scoriae, more
or less irregular in thickness, but dipping uniformly on all
sides away from the vent, with an inclination correspond¬
ing exactly to the external slopes of the mountain.
" The old crater of Somma, whose steep walls now half
encircle the cone of Vesuvius (see fig. 6), was formed by
the celebrated eruption of the year 79, which occasioned
the death of the elder Pliny, and buried Herculaneum,
Fig. 6. View of Vesuvius from the south-east, looking over
the plain of Pompeii.
Pompeii, and Stabiae beneath a bed of ashes and fragmen¬
tary scoriae from 30 to ICQ feet in thickness."
The original cone of Vesuvius in pre-historic times, long
before the eruption of 79, may have had a height of 7,000
feet,' but the mountain upon which the ancient Romans
looked was a huge truncated cone about 4,000 feet high.
The summit of this mountain enclosed a circular depressed
plain nearly three miles in diameter, within which the
gladiator Spartacus with his followers was besieged by a
' See Johnston-Lavis, " Quart. Journ. Geol. Soc.," vol. xi. p. 40.
chap. iii.j
VOLCANOES.
37
Eoman army. It does not appear that the volcanic
character of Vesuvius was then recognized; the Eoman
writers describe its slopes as bearing fertile fields and rich
vineyards, while the hollow at the top was a waste over¬
grown with wild vines. The explosive eruption of a.d. 79
blew out the southern side of this great crater, and the
present cone, which rises to about 1,000 feet above the
Atrio del Cavallo, has been gradually built up by the
successive eruptions of eighteen centuries, though in the
course of that period it has several tim^s been partially
destroyed and again reconstructed.
" The eruptions of Vesuvius seem very rarely to have
taken place from any other than the central vent ; a few
small cones immediately above Torre del Greco, thrown up
in 1794, and the cone on which the Camaldoli della Torre
is built, are the only indications of explosions having burst
from the sides of the mountain. The vast number of
vertical basaltic dykes which intersect the horizontal beds
observable in the broken cliffs of the old crater (Atrio del
Cavallo), bear witness, however, that the lava was not so
frequently elevated to the summit of the mountain with¬
out occasioning numberless cracks and rents in its internal
structure."
When Italy was first colonized by the Greeks, Vesuvius
was in a quiescent or dormant state, while the neighbour¬
ing isles of Ischia and Procida were the theatres of con¬
stant earthquakes and explosions : but since Vesuvius has
resumed its activity, Ischia has been almost entirely dor¬
mant. Ischia has numerous cones; the central one, Epomeo,
is 2,600 feet high, and has traces of two large craters on
its summit ; but eruptions have burst out at various points,
and a lava-stream that issued from its base still exhibits an
arid and cindery surface.
The Lipari Isles between Naples and Sicily, lying, as it
were, midway between Vesuvius and Etna, present a
character very analogous to the district above described.
The crater of one of the islands, Stromboli, has been in
constant activity from the earhest historical period. It
always contains melted lava in constant motion, and at
uncertain intervals the molten mass suddenly rises, and
large bubbles appear, which, upon reaching to the brim of
38
DYNAMICAL GEOLOGY.
[sec. i.
the crater, explode with a sound resembling thunder ;
masses of lava, with dust and smoke, are thrown into the
air, and the incandescent fluid then sinks down to its
former level.
The cliffs of St. Calogero, which are about 200 feet high,
and extend four or five miles along the coast, consist of
horizontal beds of volcanic tuff, traversed in every direc¬
tion by dykes and veins of lava.
Etna.—This volcanic cone rises majestically to the height
of more than two miles (10,872 feet), the circumference of
its base exceeding 180 miles. Compared with this pro¬
digious mass of igneous products, Vesuvius sinks into in¬
significance, for whUe the lava-streams of the latter do not
exceed seven miles, those of Etna are often from 15 to 80
miles in length, and from 50 to 100 feet in thickness.
The surface of Etna presents three distinct regions ;
around the lower slopes the country is richly cultivated,
and abounds in vineyards and pastures, towns and villages ;
the middle or temperate zone above is covered with forests
of oak and chestnut ; but upwards the trees grow sparse
and small, and the last 5,000 feet form a bare and sterile
region, while the summit is capped with eternal snow.
As a volcano Etna possesses two remarkable featimes—
1. The number of minor cones which have been thrown up
on its flanks, no fewer than 200 "such lateral vents being
known. In 1809 twelve new craters opened about half¬
way down the mountain, and threw out copious streams of
lava; again, in 1811, other vents appeared on the eastern
side.
2. But by far the most interesting feature of Etna is an
immense depression or excavation on the eastern side of
the mountain, called the Val del Bove. This vast hollow
is 5 miles across, and its sides are precipices from 2,000 to
8,000 feet in height.
There is good reason to believe that the Val del Bove
contains the site of a second focus of eruption, and that
the materials which once occupied it were blown away by
explosions similar to those which removed the central
portion of Somma, before the modern cone of Vesuvius
was built up. If so, the new cone, which forms the present
summit of Etna, did not rise within the old crater, but
CHAP. III.J
VOLCANOES.
39
quite outside its limits. Other cases, in which the volcanic
focus has shifted its position, are not rare, and examples
are found in Vulcano and Vulcanello, two of the Lipari
islands.^
The precipitous sides of the Val del Bove are everywhere
seamed by vertical dykes, which not only intersect the con¬
centric sheets of lava and tuff, but, standing out in bold
relief, like prodigious buttresses, impart a most extraor¬
dinary character to the scene. These buttresses are from
two to twenty feet in thickness, and their superior hardness
has enabled them to resist the action of rain and frost
better than the less coherent tuffs around them.
Professor Blake remarks ^ that if the directions of these
dykes are prolonged, the points of their intersection would
lie within the area of the Val del Bove. This, and the
fact that numerous eruptions have issued from the floor of
the valley, conflrm the view that it is a kind of crater.
Lyell describes the plain of Trifolietto, as the floor of the
valley is called, " as more uneven than the surface of the
most tempestuous sea " ; but Professor Blake says " the
great lava stream of 1852, erupted since he saw the spot,
must have entirely changed the aspect. The fluid mass
must have been kept up by a ridge of rock extending from
Cavanno on this side to Monte Penoccio on the other, and
have settled itself into a smooth and lake-like surface, over
which the winds have spread a coating of the finest dust."
At the further end of this plain are two tuff cones from
which small lava-flows have issued, and below them are
the two boceas, or low lava-mounds, whence more liquid
and voluminous flows like that pf 1852 have issued. " It
is very instructive to have the two forms of eruptive centres
thus side by side, and to compare the effects. The results
produced by the cinder cones are confined to their imme¬
diate neighbourhoods, but by comparison with the huge
lava-sheet which has overwhelmed all the country as far as
Zaffarana, the Bocca is quite insignificant. This is impor¬
tant to remember when we find a difficulty in identifying
the centre of ancient lava-flows."
^ See "Volcanoes," by J. W. Judd, for illustrations of this and
other features of volcanoes.
" Proc. Geol. Assoc., vol. xi. (1889), p. 169.
40
dynamical geology.
[sec. i.
Hawaii.—The Sandwich Islands in the North Pacificare
the best known examples of volcanoes formed by the well¬
ing up of lava without explosive action. They are all vol¬
canic, and all exhibit similar features, though only one of
them (Hawaii) now has active vents. Recent soundings
have disclosed the fact that these islands are the summits
of a gigantic submarine mountain chain, the loftier peaks
Fig. 7. Map of the island of Hawaii, showing some of the recent
lava streams. The dotted lines are contours of 2,000 feet
interval.
of which are not far off 30,000 feet above their sub-oceanic
bases. The depth of the water within thirty or forty miles
of Hawaii varies from 14,000 to 16,000 feet, and the
Hawaian volcanoes rise to nearly 14,000 feet above the
sea.
Hawaii comprises four volcanic centres, one of which is
CHAP. III.J
VOLCANOES.
41
apparently extinct (Manna Kea) ; but three (Manna Loa,
Hnalalai, and Kilanea) are still active. As already stated
(p. 24), the most notable characteristics of their eruptions
are (1) the comparatively quiet uprise of the lava and the
consequent absence of fragmentary ejecta; (2) the great
liquidity of the lavas. To these causes the low, broad out¬
line of the mountains is due (see fig. 3). The positions of
these centres, and some of the more recent lava-flows, are
shown in the plan, fig. 7.
Mauna Loa, the " Great Mountain," is 13,700 feet high,
and its summit is a nearly level plateau, in the centre of
which is a huge pit three miles long and a mile and three-
quarters broad in the middle. The floor of this pit is in
two parts, the outer ends being about 600 feet below the
rim, while the central section is about 180 feet lower, as if
it had sunk down bodily in consequence of the removal of
support below. Many eruptions have taken place from
this mountain, but the lava has generally issued from
some rent below the summit, and seldom from the crater.
The lava seems to rise in a central pipe, and to exert such
a pressure on its walls that they give way before it can
reach the summit. Thus in 1852 there was first a small
issue of lava near the summit, and then a larger outburst
from a point about 4,000 feet below it.
In April, 1868, there was an outflow of lava from the
southern border of Mauna Loa, at a point only 3,700 feet
above the sea. This was preceded by earthquake shocks,
which continued for two weeks. The lava was throvra up
in a series of jets or fountains at intervals along a line of
fissure, and the lava flowed with such rapidity that in a
little over two hours the stream had reached the sea, a dis¬
tance of 10 or 11 miles.
In February, 1877, there was an eruption of six hours
duration near the summit of the mountain ; lava-fountains
were again formed, and a representation of them is given
in fig. 8. It was followed by a submarine eruption in
Kealakekua Bay. Other more extensive eruptions occurred
in 1881 and 1887, producing lava-flows thirty miles in
length.
Kilauea (pronounced Kilouea) has usually been de¬
scribed as a subsidiary vent of Mauna Loa, situate on the
Fig. 8. Eruption of Mauna Loa, Hawaii, on February 14, 1877. After a sketch by M. Ballieu,
French Consul at Honolulu. Viewed from the north-west.
CHAP. III.]
VOLCANOES.
43
west flank of that mountain, but Captain Dutton ' has re¬
cently given good reasons for dissenting from this view,
and for regarding it as a distinct volcano, having no more
connection with Mauna Loa than with either of the other
two volcanic centres. He points out that the ground falls
slightly from Kilauea towards Manna Loa, as well as in
every other direction, though the lavas from both centres
have naturally tended to obliterate the intervening depres¬
sion. Again, Kilauea is about as far from the summit of
Fig. 9. View of the crater of Kilauea.
Mauna Loa as that is from the other two mountains.
Finally, he found that the real centre or apex of the
Kilauea dome lay some 4 or 5 miles east of the great
pit which bears that name, and that numerous old fissures
radiate from this focus. Kilauea, therefore, is an inde¬
pendent mountain, smaller than, but similar to, Mauna
Loa, and its present focus or crater lies a little west of its
apex, and at a level of 4,200 feet above the sea.
Kilauea is a great pit, from 2^ to 3^ miles wide, and about
9 miles in circumference. It is fiUed with liquid lava, and
' U.S. Geol. Survey, 4th Ann. Keport, p. 120.
44
DYNAMICAL GEOLOGY.
[sec. i.
after an eruption it is often 1,000 feet deep ; the ordinary
action in this pit consists of small ebullitions, producing jets
of lava which play like fountains to a height of 30 or 40
feet. Sometimes there are only two or three active pools,
but in times of greater activity the pools overflow, and the
pit fllls to within 400 or 500 feet of the rim. An eruption
may then be expected ; an addition of 400 feet to the
column of lava within the crater causes a corresponding
increase of pressure, and eventually one or more fractures
are produced below the crater, through which the lava
makes its way to the surface. During the eruption of
1840 the lava first rose to a great height in Kilauea, and
then broke out at several points, one below the other, finally
issuing in a continuous stream at a place about 10 mUes
from the sea, into which it flowed at Nana wale.
Volcanic Islands and Submarine Volcanoes.—
A volcanic island may have originated in two ways, it may
be either the highest peak of a sunken tract of land, and
therefore an island, because the sea has overflowed the
ground on which it stands ; or, it may have been built up
from the sea-bottom, repeated eruptions increasing the
height of the volcano till it lifts its head above the waves
and becomes an island.
An interesting example of the latter kind of volcanic
island was formed in the Mediterranean about 30 miles off
the south-west coast of Sicily during the summer of 1831,
at a spot where previous soundings had ascertained the
depth of the sea to be 600 feet. Its formation was pre¬
ceded by a violent spouting up of steam and water, and the
sea around was covered with floating cinders and shoals of
dead fish. At length a small island gradually appeared,
having a crater on its summit which ejected steam, ashes,
and scoriae. This crater attained an elevation of nearly
200 feet, with a circumference of about 3 miles, having a
circular basin full of boiling water of a dingy red colour.
The island received various names, but is best known by
the English one of " G-raham's Island," and the French one
of " risle Julia." It continued in activity for three weeks
and then gradually disappeared. In 1833, two years after
its destruction, a dangerous reef remained ll feet under
water, in the centre of which was a black volcanic rock
CHAP. III.J
VOLCANOES.
45
(probably the remains of the plug of lava in the crater),
surrounded by shoals of scorise and sand.
A similar island has recently been formed in the Pacific,
near the Friendly or Tonga Islands, and has been named
Falcon Island. The following is an outline of its history.'
In 1867 H.M.S. " Falcon " reported a shoaj 30 miles west of
Namuba Island. In 1877 smoke was reported by H.M.S.
" Sappho " to be rising from the sea at this spot. On Oct. 14,
1885, a volcanic island rose from the sea during a submarine
eruption, and was reported to be two miles long and about
Fig. 10. Graham's Island in 1831, from a sketch by M. Constant
Prévost.
250 feet high. The U.S.S. " Mohican " passed it in 1886,
and gave its length as 1-^- mile, height 165 feet ; " the
crater was on the eastern end, and dense columns of smoke
were rising from it."
The island and its surroundings were carefully surveyed
by H.M.S. " Egeria " in 1889, it was then 1^ mile long and
-1%^ of a mile wide. The southern portion is high, and faced
by cliffs, which reach to 153 feet above the sea; the northern
part is low and flat. The island seems to be formed entirely
of ashes and cinders, with a few ejected blocks and volcanic
^ See Capt. Wharton in " Nature," Jan. 23rd, 1890, p. 276.
46
dynamical geology.
[sec. i.
bombs here and there. This loose material is being rapidly
removed by the waves raised by the constant south-east
winds, and its existence as an island will probably be short,
unless it contains a hard core or plug of lava. The crater
has already been destroyed, and a shallow flat covered by
less than 6 fathoms of water extends for a mile to the south
of the island. The soundings show that it is separated from
Namuka Island by a valley 6,000 feet deep.
Metis Island, 73 miles N.N.E. of Falcon Island, is another
volcanic cone that appeared a few years before the latter,
but has not yet been surveyed.
Many of the solitary volcanic islands which occur in the
midst of the Pacific and Indian Oceans, such as Barren
Island (fig. 4), Isle of Bourbon (Eéimion), and St. Paul's
Island, may have originated in the manner above de¬
scribed.
The following is Jukes' description of St. Paul's : " This
island is 8 or 4 miles across, with a curved flat-topped ridge
820 feet high, nearly surrounding a circular crater, into
which the sea now flows from one side, and which at the
sea-level is almost half a müe in diameter. From the summit
of the circular ridge the island slopes gently down towards
the sea on all sides, except the [north-]east, where there are
vertical cliffs, formed by the sea having cut into the centre
of the island so as to gain access to the crater The
entrance was not more than 100 yards wide, and only just
deep enough for a boat, but inside there was a depth of 30
fathoms, with a bottom of black mud At several
parts of the beach hot smoking water trickled through the
stones. A bank of soundings stretched off the eastern side
of the island for a distance of nearly a mile, and a tall de¬
tached pinnacle of rock rose from this near the entrance to
the crater. This bank was evidently the base from which
the rocks that once surrounded the crater, and completed
the island, had been removed. The island seemed wholly
composed of dark lava in irregular layers, with beds of sand,
ashes, and blocks, varying from black to red and cream-
coloured. They dipped but slightly outwards, and in one
part seemed to dip inwards or towards the crater." '
* Jukes, " Manual of Geology," second edition, p. 336.
CHAP. III.]
VOLCA^"OE.S.
47
Fig. 11 is a map of this curious island, with submarine con-
tonrs at intervals of ten fathoms.
The small islands of Santorin and Therasia in the .®gean
Sea, have been explored by Admiral Spratt and Professor
E. Forbes (1841), and more recently by M. Fouque (1880).^
These two islands form the crater rim of a partially sub¬
merged volcano, which is still occasionally active; the
Fig. 11. Map of the Island of St. Paul's.
modern eruptions proceeding from small islands thrown up
in the central lagoon. Marine shells have been found in
the tuffs of Santorin up to a height of 600 feet above the
sea, proving that elevation to that extent has taken place
since volcanic action began.
During the gradual building up of such a submarine
volcano, the lava-streams and fragmental materials which
^ See Lyell's "Principles," tenth edition, vol. ii. p. 66, and
Fouqué's "Santorin et ses Eruptions," Paris, 4to, 1880.
48
dynamical geology.
[sec. i.
are ejected into deep water are probably arranged in much
the same manner as those which fall on land. Some oppor¬
tunities have occurred for observing the flow of lava
beneath water, and it has been noticed that the crust which
rapidly forms over the surface prevents the water from
coming in contact with the internal fluid portion of the
stream. The escape of heat is also checked by the low
conducting power of the crust, and so the interior remains
fluid, and continues to flow onward, just at it would do on
land. Professor Green has even surmised that, "if the
discharge takes place in deep water, it is conceivable that
the pressure of the overlying fluid will check the escape
both of elastic fluid and of heat from the lava, and so
keep it fluid for a longer time, and cause it to spread out in
wider and more regular sheets, than if it had flowed out in
the open air." '
Again, the ash and cinders ejected from such submarine
vents will probably arrange themselves on the slopes of
the cone, and form beds of tuff and agglomerate, similar
to those of sub-aerial origin. But the case is different
with the ejections from volcanic islands, because such of
these as do not fall on the slopes of the island itself will
drop into the shallow waters surrounding it, and will be
subjected to the distributive action of the sea-waves, tides,
and currents. They will be arranged and stratified on the
sea-bottom, and will often be mixed with the other deposits
which are there being formed. Arenaceous or calcareous
tuffs may thus be produced by the admixture of sand or
limestone. Moreover, in all such submarine tuffs, fish,
shells, and other marine remains are liable to be enclosed,
and will testify by their presence to the conditions under
which the stratified masses have been accumulated.
* Green, " Physical Geology," second edition, p. 225.
CHAP TEE IV.
EAETHQTTAKES,
ONNECTION between Volcanoes and Earth¬
quakes.—It has been mentioned that before the
bursting out of any great volcanic eruption the neighbour¬
hood is generally shaken by earthquakes. It has also been
observed that a succession of earthquakes in a district is
often closed by a great volcanic eruption, either in that or
in some neighbouring district. These facts suggest that
the earthquakes form part of a series of subterranean dis¬
turbances which culminate in a volcanic eruption, and
that the shocks are due to the exertion of the same force
which opens an upward way for the lava till it can be
ejected from a volcano. It is not unlikely that both earth¬
quakes and volcanic phenomena are due to the forcible
opening of cracks in the earth's crust, the crack starting
as a rent or fissure at a great depth.
The connection between volcanoes and earthquakes was
exhibited in a remarkable manner by the series of events
which took place in and around the Caribbean Sea and
G-ulf of Mexico at the beginning of this century. From
May, 1811, to April, 1812, severe earthquakes were felt in
the West India Islands, especially in St. Vincent ; violent
shocks also occurred in the Mississippi valley, and at
Caraccas in Venezuela. Finally, on April 30th, 1812, the
volcano of St. Vincent, which had been quiescent for nearly
a hundred years, burst into eruption ; and so tremendous
was the explosion that the noise of it was heard at Caraccas,
nearly 500 miles away. This eruption seemed to relieve
the subterranean tension, and the earthquakes by which
the surrounding regions had been so greatly shaken then
50
DYNAMICAL GEOLOGY.
[sec. i.
ceased. As the area affected in this case was about 2,000
miles in length, and at least 500 wide, we may infer that
the volcano of St. Vincent is connected, not merely with a
local reservoir of lava, but with a very large subterranean
tract, or, as is most probable, with the liquid substratum
which is supposed to underlie the earth's crust (see p. 17).
The connection between the two sets of phenomena is
also shown in other ways. Mr. Mallet,' who first studied
the extent of the areas which are most frequently and
violently disturbed by earthquakes, found that these areas
form bands of great but variable breadth, which he called
seismic bands.'
He prepared a map of the world on which the districts
known to have been shaken by earthquakes were coloured
brown, the tint being made darker and darker in propor¬
tion to the frequency and intensity of the shocks which
had been experienced at different localities.
A glance at such a map shows that the bands of darkest
colour run along the mountain chains on which volcanoes
occur ; and Mr. Mallet has formulated the following con¬
clusions :—
1. These seismic bands very generally follow the lines
of elevation which mark and divide the great oceanic or
terroceanic basins of the earth's surface.
2. In so far as these are frequently the fines of moun¬
tain chains, and these latter often include fines of volcanic
vents, so the seismic bands follow them likewise.
3. The sensible width of the seismic band depends upon
the energy developed, and on the accidental geological and
topographical conditions along its length.
4. Seismic energy may become sensible at any point of
the earth's surface, its efforts being, however, greater as
the great volcanic lines of activity are approached.
Professor G. Darwin has pointed out ' that an earth¬
quake map also suggests the inference that there is a
broad band of seismic action which completely encircles
' " Catalogue of Earthquakes," Brit. Assoc. Reports, 1847 to
1861.
From seísmos, an earthquake (Greek) ; hence also seismology,
the study of earthquakes.
' " Fortnightly Review," Februaiy, 1887.
CHAP. IV.J
EARTHQUAKES.
51
the glohe. This traverses Southern Europe and the Medi¬
terranean, Asia Minor and Syria, Persia, India, China, and
Japan; crosses the Pacific to Central America and West
Indies, and the Atlantic to the Azores, Teneriffe, Portugal,
Spain, and N.W. Africa. Although the intensity of the
earthquakes felt along this band varies in different parts
of it, yet he thinks the evidence is sufficient to show that
there is an annular tract of the earth's crust which is
specially liable to rupture and consequent disturbance.
The other seismic bands, such as those of the Andes and
the Malay Archipelago, may be regarded as offshoots more
or less at right angles to the main band.
Nature and Origin of Earthquakes.—Mr. Mallet ^
defines an earthquake as a wave of elastic compression, or
a succession of such waves travelling through the solid
substance of the disturbed country, and it is the emer¬
gence of such a wave at the surface of the ground which
causes the actual earthquake shock.
This wave is transmitted from a subterranean centre of
impulse or focus (a in fig. 12), which Mr. Mallet treats as a
rent or fissure, and it proceeds from that focus with equal
velocity in every direction. The line along which the wave
reaches the surface directly over the centre of impulse {a h),
Mr. Mallet calls the seismic vertical, b being called the
epicentrum or point directly over the centre of impulse,
and the concentric lines along which the shock reaches the
surface at the same moment he calls the coseimal lines (c c,
d d) ; the wave strikes the surface more and more ob¬
liquely as we recede from the line, a h, and with less and
less force, till it gradually fades away.
In fig. 12, the centre of impulse is represented as a point,
and it is assumed that the surrounding rocks are homo¬
geneous, so that the coseimal lines are circles. In reality,
however, the centre of impulse must generally be a narrow
fissure of some length, and the impulse will generate waves
proceeding outwards from the surface of the fissure in
ellipsoidal shells, the coseimal lines being ovals whose
major axes are at right angles to the plane of the fissure.
The variation in the density and elasticity of the rocks
' " First Principles of Observational Seismology."
52
DYNAMICAL GEOLOGY.
[sec. i.
througli which the wave passes, also interferes with the
regularity of the coseimal lines, and the passage of the
wave from one kind of rock to another causes refractions,
so that the actual form of the wave at the surface is com¬
plex and irregular.
The earth-wave travels most rapidly through hard and
compact rocks, less rapidly through soft and incoherent
beds. Mr. Mallet has experimented by exploding gun¬
powder and calculating the velocity of the shock so pro¬
duced in different rocks. He found that the wave tra¬
versed—
Solid granite at a rate of 1,665 feet per second.
Shattered granite „ 1,306 „ „
Slate „ „ 1,089 „ „
Wet sand „ „ 825 „ ,,
By observing the direction of fissures and by the use of
instruments called seismometers, Mr. Mallet believed it
possible to determine the direction and the angle of emer¬
gence of the wave-path, and thus to calculate the position
and depth of the centre of impulse from which the wave
proceeded. By this method Mr. Mallet calculated that the
centre from which the shock of the great Calabrian earth¬
quake in 1857 proceeded, was beneath a spot about 60
miles E.S.E. of Naples, and at a depth of about 5 miles
from the surface. He assigned 30 geographical miles, or
185,000 feet, as the limit of depth at which any earthquake
originates.
CHAP. IV.J
EARTHQUAKES.
53
It has, however, been pointed out that the passage of a
shock through loose soils and surface rocks cannot be com¬
pared with that through dense, compact, and deeply buried
rocks ; and that the wave is chiefly propagated through
such deeply buried rocks. The actual rate of propagation
must depend on the density and elasticity of the materials
through which it passes ; the rate of the passage of com-
pressional waves through a bar of steel has been estimated
at 21,000 feet per second, and by comparing the density
and elasticity of very dense rocks with the same properties
of steel, the rate of passage through such rocks has been
calculated to be about 18,400 feet, of 3-4 miles per second.^
It is a remarkable fact that in the case of the Charleston
earthquake of 1887 six careful time records, taken at dis¬
tances of 300 to 645 miles, gave an average result of 3'3
miles per second as the rate of speed. This is a much
higher speed than has hitherto been observed in European
earthquakes, which have given results varying only from
660 to 2,860 feet per second. The discrepancy remains to
be accounted for ; it seems too great to be explained by
errors of observation, and the speed of the Charleston
earthquake wave may possibly be due to the greater depth
of its point of origin.
It is not always possible, however, to ascertain the angle
of emergence, and another method has recently been sug¬
gested by the investigators of the Charleston earthquake,
which promises to give more accurate results. By this
method the depth of the seismic focus at Charleston in
1887 was calculated to be about 12 miles.
Messrs. Button and Hayden believe that few earthquakes
originate at a greater depth than this. They point out
that earthquakes differ in energy and in depth of origin,
the intensity of the shock at the surface depending on the
energy developed at the focus, and on the distance of that
focus from the surface. An earthquake of great intensity
may be due to the focus being near the surface, or to the
development of great energy at the focus, or to both com¬
bined. It follows also that the radial distance to which
' " Investigation of the Charleston Earthquake," by C. E.
Button and E. Hayden, " Nature," July 28th, 1887, p. 302.
54
DYNAMICAL GEOLOGY.
[sec. i.
the shock is transmitted is the best measure of the total
energy of the shock. If, therefore, in comparing the effects
of two earthquakes, we find that the intensity of the shock
at the epicentrum was about the same, but that one was
felt over a much wider area than the other, we may at
once infer that the focus of the one was more deeply seated
than that of the other. Now the Charleston earthquake
was felt at a distance of 1,000 miles from the centre, and
yet its intensity at the epicentrum was relatively low ;
hence these observers conclude that the depth at which it
originated was relatively great. This depth they found to
be about twelve miles; they therefore regard this as a
great depth, and suggest that estimates of the depth of
earthquake foci exceeding this require re-investigation.
("Nature," July, 1887, p. 300.)
Phenomena and Effects of Earthquakes.—The
phenomena attendant upon an earthquake shock vary ac¬
cording as the centre of impulse is under the land or under
the sea. In the first case waves are only progagated
through the earth and through the air : in the second case
the waves are communicated to three media, viz., earth,
air, and water. Moreover, sound-waves appear to be
carried through the earth and the sea, as well as through
the air, so that if the shock originates under the sea, an
observer standing near the shore will perceive the follow¬
ing succession of phenomena.
1. A rumbling noise, heard apparently through the feet ;
this being the sound-wave through the earth, which is said
to travel at a rate of 11,000 feet per second.
2. Another sound caused by the sound-wave through the
air, which travels at the rate of 4,700 feet per second.
3. The earth-wave, bearing with it a small forced sea-
wave ; its velocity varying, as already stated, from about
800 to 1,600 feet per second (mean rate about 1,200 feet).
4. A third sound produced by the sound-wave through
the water, which is carried 1,138 feet per second, and there¬
fore is sometimes heard at the same time as the earth-wave
is felt.
6. The great sea-wave, the sea being first sucked back
for some distance from the shore, and then returning as a
wall of water from 15 to 20 feet high, rolling in far over
CHAP. lY.J
EARTHQUAKES.
55
the land and washing down everything before it. The
first is often followed by smaller waves.
The velocity of the sea-wave depends upon the depth of
the water through which it is propagated ; it travels
through deep water much faster than through shallow
water near the land. Thus the sea-wave of the Lisbon
earthquake in 1755 travelled to London at a little over 2
miles a minute, but crossed the Atlantic to Barbados at a
rate of about 5 miles per minute. The wave which travelled
across the Pacific from Japan to California in 1854 had an
average velocity of 6T miles per minute.^
The surface efiEects of an earthquake wave are more
destructive when it traverses soft rocks because the cracks
which it produces at the surface are kept open for a longer
time, and allow buildings to subside ; while in hard rocks
these fissures are narrower and close more quickly, so that
less displacement is caused. When the wave passes from
compact into loose rocks, there is often a complex reflection
and reverberation of shocks which results in a destructive
shivering of the surface.
If the angle of emergence is low, the difficulty which the
wave finds in passing from hard into overlying soft rocks
is such that a very small shock is propagated through the
latter. Thus the Lisbon earthquake was felt in Scotland,
but not in England, and the reason is supposed to be that
the wave travelled through the hard rocks at a great depth
below the ground, and the slight shock communicated to
the thick mass of softer rocks which form the greater part
of England died away before it reached the surface.
Instances of Earthquakes. — Italy. — The earth¬
quakes which convulsed Calabria in 1783 were remarkable
for the number of separate shocks (949) that occurred
during the year, and for the great disturbances and changes
which they produced on the surface of the ground. The
area over which these shocks exerted their most violent
effects was not more than 500 square miles, but they were
peeceptible over a great part of Sicily, and as far north as
Naples.
^ From this it has been calculated that the mean depth of the
body of water through which it moved, viz., the North Pacific,
was 14,190 feet.
56
DYNAMICAL GEOLOGY.
[sec. j.
In some of the Calabrian towns the pavements were
thrown into the air and reversed so as to lie bottom up¬
wards. Long undulating fissures were formed, some of
which remained open, and some gradually closed up again.
The wells and springs were violently disturbed, and in
some places circular cavities were opened from which water
escaped, and which afterwards remained in the form of
pools. ^
In the town of Terranuova some houses were elevated
above their former level, while others sank below it, and
one tower of solid masonry was divided into two parts by a
crack, on one side of which the building was elevated
several feet above the other part. This vertical shift was
rendered apparent by the discontinuity of the courses of
stone, which presented a counterpart of the dislocation
which must have been produced in the rocks below, and
which is termed a, fault by geologists.
Great mases of earth and rock slid down from the sides
of the valleys, and some of these landslips dammed up the
rivers, and gave rise to new lakes. Thus enormous masses
of land were detached from each side of the deep valley or
ravine of Terranuova, and blocked up the course of the
river, causing the formation of a large lake. It is stated
that about fifty such lakes were formed at this time ; some
of these were permanent, the course of the streams being
altered, but others were gradually drained by the water
overflowing and cutting a channel through the barrier.
Along the sea-coast huge masses were detached from the
cliffs, especially at Scilla and at Gian Greco in the Straits
of Messina, where a continuous line of cliff was thrown
down for the length of a mile.
The Neapolitan earthquake of 1857 was one of great
intensity, the shocks causing much displacement of the
ground, which was in many places rent and fissured. Some
of these fissures remained open after the cessation of the
shocks, and fig. 13 represents a fissure observed by Mr. E.
Mallet in the gorge of Bella near Naples after that earth¬
quake. It will be observed that it exhibits a certain
amount of vertical movement, one side of the fissure being
higher than the other.
' See the account in Lyell's " Principles," vol. ii. ch. xxix.
CHAP. IT.J EARTHQUAKES. 57
South America. — In the earthquake of Quito, ou
February 4th, 1797, the velocity of the shock was so great
that Humboldt says the " explosive movement was such as
Fig. 13. View of a fissure in the Gorge of Bella, Naples, 1857.
is produced by the firing of a mine, and the bodies of
several of the inhabitants were thrown upon the hill of La
Cullca, which rises on the other side of the Licau torrent."
The actual range of vertical projection of these bodies has
58
DYNAMICAL GEOLOGY".
[sec. i.
been calculated, according to Mr. Mallet, at 100 feet, which
he says would give a velocity of shock equal to 80 feet per
second.
The centre of the disturbance seems to have been the
volcano Tunguragua, and the district most violently shaken
measured 120 miles from north to south, and 60 miles from
east to west, but the shocks were felt far beyond these
limits. Riobamba and all the other towns within this
area were destroyed. The ground about Tunguragua
opened into enormous clefts, from which issued volumes of
water and stinking mud, forming lakes in many places of
considerable size.'
Another of the great earthquakes which so frequently
shake the western coast of South America happened on
the 20th of February, 1835. It was felt in ¿1 places
between Chiloe on the south and Copiapo on the north, a
distance of more than 1,000 miles, and between the city of
Mendoza on the east and the island of Juan Fernandez on
the west, a distance of at least 600 miles. Admiral Fitzroy,
who was at Talcahuano, the port of Concepción, says that
after the earthquake there was a belt of coast 4 or 5 feet
in height, which, even at high water, shewed beds of dead
mussels, limpets, and withered sea-weed, aU still adhering
to the rocks. This raised beach gradually sank again until
the part above high-water mark was not more than 2 feet
high. He also visited the neighbouring island of Santa
Maria, where he found that the following changes had
taken place. " It appeared that the southern extremity of
the island had been raised 8 feet, the middle 9, and the
northern end upwards of 10 feet. On steep rocks, where
vertical measures could be correctly taken, beds of dead
mussels were found 10 feet above high-water mark. An
extensive rocky flat lies around the northern parts of Santa
Maria. Before the earthquake this flat was covered by the
seaj some projecting rocks only showing themselves. Now
the whole flat is exposed, and square acres of it are covered
with dead shell-fish, the stench arising from which is
abominable." '
' Humboldt's " Voyage," p. 317.
' Phil. Trans., 1826, and LyelTs " Principles,'' tenth edition,
vol. ii. p. 93.
CHAP. lY.J
EARTHQUAKES,
59
Speaking of the same earthquake the late Dr. Darwin
says : ^—" The island of Quiriquina plainly showed the over¬
whelming power of the earthquake. The ground in many
parts was fissured in north and south lines, perhaps caused
by the yielding of the parallel and steep sides of this narrow
island. Some of the fissures near the cliffs were a yard
wide. Many enormous masses had already fallen on the
beach ; and the inhabitants thought that when the rains
commenced far greater slips would happen. The effect of
the vibration on the hard primary slate, which composed
the foundation of the island, was still more curious : the
superficial parts of some narrow ridges were as completely
shivered as if they had been blasted by gunpowder. This
effect, which was rendered conspicuous by the fresh frac¬
tures and displaced soil, must be confined to near the sur¬
face, for otherwise there would not exist a block of solid
rock throughout Chile; nor is this improbable, as it is
known that the surface of a vibrating body is affected dif¬
ferently from the central part. It is, perhaps, owing to
this same reason, that earthquakes do not cause quite such
terrific havoc within deep mines as would be expected. I
believe this convulsion has been more effectual in lessening
the size of the island of Quiriquina, than the ordinary
wear-and-tear of the sea and weather during the course of
a whole century."
North America.—The earthquake which originated near
Charleston (Carolina) in 1887 has been carefully investi¬
gated by Capt. Dutton and Dr. Hayden, and some of their
observations have already been mentioned. This earth¬
quake was felt at a distance of 1,000 mües in more than
one direction, but there were several tracts where the
intensity of the shock was feeble compared with its effect
on the surrounding areas. Two of these are quite isolated,
the third is a tongue-shaped tract over the north-west part
of the Appalachian region in Virginia. The cause of
these tracts being less shaken is not clear. The tract over
which the most forcible shocks occurred was an elliptical
area about 26 miles long and 18 wide ; the major axis of
this was not a straight line, but a ciHve, with its concave
' " Voyage of the Beagle," edition of 1860, p. 393.
60
dynamical geology.
[sec. 1.
side towards Charleston, and lying about 14 miles from
that city. Along this line there were three points which
had the characters of epicentra, indicating apparently
three distinct foci or centres from which three separate
shocks proceeded. One of these was computed to lie at
about 12 miles from the surface, and the others appeared
to be at the same depth.
New Zealand.—A great earthquake occurred in New
Zealand in 1855, during which a tract of land comprising
4,600 square miles is believed to have been permanently
upraised ; Sir C. Lyell states on the authority of Mr.
Edward Roberts, R.E.,' that the amount of elevation in¬
creased eastward and attained its maximum along the
Rimutaka range near Port Nicholson, where the elevation
was 9 feet. The land on the west side of these mountains,
Fig. 14. Fracture and upheaval in New Zealand, after LyeU.
forming the plain of Wairarapa, was not raised at aU, the
vertical movement ceasing abmptly along the base of the
hills, and forming a line of fault or fracture which was
traced for a distance of 90 miles.
The course of the fracture along the base of the hills
was rendered visible by a nearly perpendicular cliff of fresh
aspect about 9 feet in height. It was marked moreover in
many places by fissures from 6 to 9 feet broad, filled here
and there with soft mud and loose earth. The relations of
the rocks on either side of this dislocation are indicated in
the diagram, fig. 14, the hard rocks a being elevated, while
the newer, b, remained horizontal. Previous to the earth¬
quake there had been no room to pass between the sea
and the base of the perpendicular cliff in which the
^ Lyell's " Principles," tenth edition, vol. ii. p. 85.
CHAP. IT.J
EARTHQUAKES.
61
Eimutaka range terminates in Cook's Strait, except for a
short time at low water ; " but immediately after the up¬
heaval a gently sloping raised beach, more than 100 feet
wide, was laid dry, affording ample space at all states of
the tide for the passage of man and beast." ^
Summary of Surface Effects produced by Earth¬
quakes.—1. Heaving of the Ground.—The peculiar motion
felt during an earthquake results from a combination of
two kinds of movements ; an upward shock or jerk, and a
progressive wave or rolling movement, and it is this com¬
bination which produces such destructive results. The
upward shock opens the cracks and joints in all rocks
as well as in the walls of buildings, and the undulating
movement rocks them to and fro, shaking down buildings
and hurling large masses from sea-cliffs and valley-sides.
2. Bamming up of Rivers.—The masses of earth and rock
thus detached from the sides of a valley often block up the
stream at the bottom, and cause the formation of a lake.
If the barrier remain, and the stream is diverted into a new
course, the lake may be permanent ; but if the dam gives
way and the pent-up waters escape, the lake is drained and
destructive floods are caused.
3. Opening of Cracks and Fissures.—Instances of such
fissures have been mentioned. They sometimes open and
shut again rapidly as the undulating movement passes on,
but frequently remain as open clefts or chasms, varying in
length from a few feet to as many miles. Sometimes one
side of the rent is at a higher level than the other, such a
dislocation forming a fault in the rocks below. Rivers are
occasionally engulfed in these fissures, and pursue an
underground course for some distance.
4. Permanent Changes of Level.—These are the most im¬
portant geological results which earthquakes produce, for,
as already stated, the solid land is sometimes raised or
lowered several feet at a time over large areas. The extent
of such movements is best seen along the sea-shore, where
in the one case a new strip of land may be added to the
coast-line by the elevation of the beach and adjacent sea-
bottom, or in the other case, if depression has taken place.
^ Lyell, op. cit. vol. ii. p. 86.
62
dynamical geology.
[sec. i.
the sea may have overflowed its ancient limits and en¬
croached upon the land. Inland the slope of water-courses
is often altered, the rapidity of the current being increased
or diminished according to circumstances, and sometimes
rivers are diverted from their former channels. An instance
of this in South America is mentioned by Dr. Darwin, on
the authority of a resident engineer.
" Travelling from Casma to Huaraz (not very distant
from Lima), Mr. Gill found a plain covered with ruins and
marks of ancient cultivation, but now quite barren. Near
it was the dry course of a considerable river, whence the
water for irrigation had formerly been conducted. There
was nothing in the appearance of the water-course to indi¬
cate that the river had not flowed there a few years pre¬
viously ; in some parts, beds of sand and gravel were spread
out ; in others, the solid rock had been worn into a broad
channel, which in one spot was about 40 yards in breadth,
and 8 feet deep. It is self-evident that a person following
up the course of a stream, will always ascend at a greater
or less inclination ; Mr. Gill, therefore, was much asto¬
nished, when walking up the bed of this ancient river, to
find himself suddenly going down hill. He imagined that
the dovmward slope had a fall of about 40 or 50 feet
perpendicular. We here have unequivocal evidence that a
ridge had been uplifted right across the old bed of a stream.
From the moment the river-course was thus arched, the
water must necessarily have been thrown back and a ne v
channel formed. From that moment also, the neigbouring
plain must have lost its fertilizing stream, and become a
desert." (" Voyage of the Beagle," edition 1860, p. 359.)
5. Alternating Movements.—One of the most interesting
examples of alternating movements of subsidence and up¬
heaval in a volcanic district is that afforded by the remains
of the celebrated temple of Jupiter Serapis at Puzzuoli on
the Bay of Baiae. These movements may not all have been
accomplished suddenly during earthquakes, but there is
reason to believe that some of them were, and though
at times the movement may have been slow and gradual
it was evidently due to the same local causes.
The ruins of this building were discovered about the
middle of last century, and excavations disclosed a square
CHAP. IV.j
EARTHQUAKES.
63
floor wliich had originally supported forty-six noble columns,
twenty-four consisting of granite, and twenty-two of fine
green marble. Only three of these remain erect, and are
httle more than 40 feet high. Up to a height of about 12
feet from their pedestals their surface is smooth and unin¬
jured, but above this level there is a band or zone, about
nine feet in height, where the marble has been perforated by
a boring mollusc {Lithodomus dactyhis), the shells of which
Fig. 15. Kemains of the Temple of Jupiter Serapis, after Lyell.
are still to be found at the bottom of the holes. These
molluscs are common in the Mediterranean, and bore their
way into limestone rocks just as the " shipworm " does into
timber.
Evidence of a still earlier subsidence was discovered in
1828 by excavations below the marble pavement on which
the columns stand, a second mosaic pavement being found
at a depth of five feet below the upper one. The existence
of this lower pavement implies a subsidence which ren-
64
dynamical geology.
[sec. i.
dered it necessary to construct a new floor at a higher
level.
The temple area was filled with a succession of stratified
deposits, consisting of marine and fresh-water limestones
with two layers of volcanic tuff, ejected, probably, from the
neighbouring crater of the Solfatara. These beds concealed
the lower part of the columns up to the base of the perfo¬
rated band, and the perforations were thus evidently made
during a subsequent subsidence of 9 or 10 feet ; the lower
portion being protected from their attacks by the accumu¬
lation of sediment, and the upper part at the same time
projecting above the surface of the sea.
The upper limit of the perforations was, in 1828, about
23 feet above the level of the sea, so that there had been
an elevation of the whole area to at least this extent. This
upheaval appears to have taken place during the 16th
century, for two documents are cited in which Ferdinand
and Isabella of Spain grant to the University of Pozzuoli a
portion of land " where the sea is drying up " (1508), and
again, " where the ground is dried up " (1511). This seems
to indicate that a slow and gradual upward movement was
taking place during this period, but it is probable that the
principal elevation took place at the time of the great erup¬
tion of Monte Nuovo in 1588. Two eye-witnesses of this
convulsion declare that the sea then abandoned a consider¬
able tract of the shore.'
The oscillations of level which appear to have occurred
in this area during historic times may therefore be thus
summarized ;—
1. Subsidence of 5 feet between the two pavements.
2. Period of rest, the temple built.
8. Gradual subsidence of 12 feet, and deposition of
sediment ; subsidence continued for 9 feet without depo¬
sition.
4. Period of rest, and perforations made in the columns
by boring molluscs.
5. Gradual upheaval to extent of more than 28 feet.
6. Slight subsidence again at the beginning of the present
century.
• " Principles of Geology," tenth edition, vol. ii. p. 174.
CHAPTER V.
ELEVATION AND DEPEESSION CE LAND.
E have already seen that movements of elevation and
depression often take place during earthquakes, and
that considerable tracts of land have been permanently
raised or lowered in this way. We have now to learn that
in some parts of the world, where no great earthquakes
have happened during the periods of history, there have,
nevertheless, been similar changes in the level of the land,
the movements often afEecting large areas of the earth's,
crust, and taking place quite gradually and imperceptibly,
not by jumps and starts as in the case of earthquakes.
There is also reason to believe that both kinds of move¬
ment may take place at different times in the same area,
slow and imperceptible changes occurring in the intervals
between the sudden shocks ; there is proof, moreover, that
intervals of rest alternate with epochs of movement, and
that sometimes movements of elevation have succeeded
movements of depression in the same area.
The signs and evidences by which we may know that a
movement of upheaval or one of subsidence has taken place
in a given district are not equally obvious in both cases.
When land is lifted up above the sea, it generally brings up
with it some proofs of its having once been under water ;
but when a coast-line is sinking below the sea-level, the fact
is less easily proved, because the subsidence soon removes
its surface from our inspection. There are, however, facts
of various kinds which are accepted as sufficient evidence of
subsidence, and others which are proofs that elevation has
taken place. It will be useful, therefore, if we tabulate the
indications which can be adduced as evidence for such
F
66
DYNAMICAL GEOLOGY.
[sec. i.
changes of level, and exemplify each kind of testimony by
appropriate instances.
Proofs of Elevation.
1. Testimony of Human Erections.—When ancient moles
and harbour works have been raised above the level of the
sea, they afford very convincing proofs of upheaval, but it
is only in countries where man has resided for many cen¬
turies that such testimony can be looked for. The island
of Crete or Candia is, however, a case in point. This
island is about 135 miles long, and Captain Spratt, E.N.,
ascertained that its western end has been uplifted 17 feet
above its ancient level, while a part of the southern coast
has risen more than 27 feet, so that the docks of the ancient
Greek ports are now above water, as well as masses of lime¬
stone rock drilled by litKodomi. At the same time, the
eastern part of the island has sunk many feet^ causing the
submergence of several Greek towns, the ruins of which are
still visible under the water.
2. Testimony of raised Roclcs and Islands.—The coasts of
Norway and Sweden consist chiefly of hard rocks, and are
fringed with numerous islets ; and as there are no tides in
the Baltic Sea, conditions are there peculiarly favourable
for making exact observations on the relative level of land
and sea.
South of Stockholm no rise is perceptible, and it appears
that there has even been depression, but north of that city
the elevation is sufiSciently obvious. It was observed early
in the last century by Celsius, who remarks that " several
rocks on the shores of the Baltic which are now above
water (a.d. 1780), were not long before sunken rocks, and
dangerous to navigators ; one especially, which in the year
1680 was on a level with the surface of the water, is 20|
Swedish metres above it (60 English feet). From an in¬
scription near Aspo, in the Lake Maelar, which communi¬
cates with the Baltic, engraved, as it is supposed, above
500 years ago, the land appears to have risen no less than
13 Swedish feet (12*66 British)." '
' Playfair's " Illustrations of the Huttonian Theory," edition
1822, p. 436.
CHAP, v.] ELEVATION AND DEPRESSION OF LAND. 67
Celsius' statements excited so much attention and discus¬
sion, that lines or grooves, together with the date of the
year, were cut in the rocks at different places along the
Baltic to indicate the ordinary level of the water on a calm
day. In 1820-21 all these marks were examined by the
of6cers of the pilot establishment of Sweden, and they re¬
ported that the level of the Baltic was certainly lower,
relatively to the land, than it had been, hut that the amount
of change had not been everywhere the same. During
their survey they also cut new marks for the guidance of
future observers.
Sir Charles Lyell visited Sweden in 1834, with the view
of determining this question, and specially examined the
marks cut on the shores of the Gulf of Bothnia. He found
that the level of the sea at that time was several inches
below the marks cut by the pilots, and from 2 to 3 feet
below the more ancient marks. Thus at Gefle, north of
Stockholm, for example, he found that the land had risen
about 4 inches in the interval between 1820 and 1834, and,
consequently, that the upward movement had proceeded at
the rate of about 2l feet in a century. He ascertained also
that the testimony of the inhabitants, both on the eastern
coasts and on the western near Gothenburg, agreed with
that of their ancestors—viz., that the low rocks, both on
the shore of the mainland and on the islands, are more and
more exposed to view.'
3. Testimony of Marine Shells.—The evidence already
cited refers only to recent historic times, but there are
other and convincing proofs that the elevation of Scandi¬
navia has been going on for many centuries, and that
similar upward movements have taken place in many
other parts of the world. The first of these proofs is the
existence of beds of sea-shells, sometimes many miles in the
interior of the country, and often at a height of several
hundred feet above the present level of the sea.
In Sweden, for example, such banks containing recent
marine shells have been met with at many localities, and
at various heights, from 10 to 200 feet. Between Stock¬
holm and Gefle there are deposits containing shells of the
' " Principles of Geology," tenth edition, vol. ii. p. 190-1.
68
DYNAMICAL GEOLOGY.
[sec. i.
same species as now inhabit the bracfeish waters of the
Bothnia Gulf up to elevations of 100 feet. Lyell traced
these beds inland to a point on the southern shores of Lake
Maeler, about 70 miles from the sea ; and M. Erdman has
subsequently traced them to Linde, a distance of 130 miles
from the sea, where they occur at a height of 230 feet.
Similar beds of sea-shells have also been found at an
elevation of 200 feet on the northern border of Lake
Wener, 50 miles from the western coast, so that a very
large area must have been added to Sweden in compara¬
tively recent times.
Uddevalla, near Wenersburg, is another place where beds
of recent shells have been found ; and here M. Alex.
Brongniart, on removing part of the shell deposit, per¬
ceived that the underlying surface was covered with bar¬
nacles still firmly adhering to the rocks. This observation
was verified by Sir C. Lyell by a similar discovery at
another spot near Uddevalla.'
In parts of Norway deposits containing sea-shells of re¬
cent species occur at elevations of 600 and even 700 feet,
according to Mr. Torell.
Dr. Darwin has described the beds of sea-shells which
he found at various heights above the sea along the west
coast of South America. Commencing at Tierra del Fuego,
he traced them for a distance of 2,075 miles along the
western coast, and at various elevations, from 300 to 1,300
feet above the sea. They occur in connection with the
raised beaches presently to be mentioned ; and Darwin re¬
marks that the shells at the lower levels were fresh, but
that those at the greater heights were brittle and decom¬
posed from prolonged exposure to the weather. Those
near Valparaiso, where he found them up to a height of
1,300 feet, were embedded in a reddish mould, having a
guano-like smell, and in which minute fragments of sea-
urchins and other marine animals could be detected.
4. Testimony of Raised Beaches.—A beach may be defined
as an accumulation of sand or shingle piled up by the action
of the waves against a shore line. Wherever the shore is
backed by a range of cliffs a beach is formed along their
' " Principles," vol. ii. p. 192.
CHAP, v.] ELEVATION AND DEPRESSION OF LAND. 69
base. If the land is elevated, both beach and cliff-line are
lifted above the reach of the waves, a new beach is formed
at a lower level, and the old beach remains as a flat shelf
or gently sloping terrace between the cliffs and the sea
margin (see fig. 17).
The width of this terrace will depend on the slope of the
shore, and the length of time during which the land re-
Fig. 17. View of a raised Sea-beach.
mained stationary. Sometimes a succession of narrow
benches or terraces may be traced round lofty cliffs, one
above another, indicating successive pauses between the
movements of upheaval by which the land was elevated to
its present height.
Scandinavia. — Such raised beaches and ancient sea-
margins form conspicuous features in some parts of Norway
and Sweden. The rise seems to have been greater towards
the east as well as towards the north, the whole country
being tilted up in a north-easterly direction. On the south-
west coast of Norway the smooth, roimded rocks slope
DYNAMICAL GEOLOGY.
[sBC. I.
gently under the sea, and no signs of upheaval are to be
seen ; but eastward, along the sides of the long fiords
which run into the land, notches in the rocks and low
terraces become visible ; and, at the east end of the Sogne
Fiord, these rise to a height of 300 feet above the sea.
Northward along the coast no signs of upheaval are met
with till the district of Drontheim is reached, where a long
low flat intervenes between the shore and the cliffs.
Beyond this point every fiord and valley has beautiful
terraces along its sides, rising northward and eastward to
levels of 200 and even 500 feet above the sea. In Finmark,
along the Altenfiord, there are terraces at successive levels
of 56, 65, and 143 feet ; but the uppermost does not main-
Fig. 18. Baised Beach forming a Terrace between Vaag and
Tromsö, on the coast of Norway, after Lehmann.
tain a constant level for, in a distance of 48 miles, one
end of it was found to be 58 feet below, and the other end
96 feet above the normal line, the north end being thus
154 feet higher than the other. Here, therefore, the
elevation during the formation of this terrace was very
unequal.
South America.—The repeated elevations which have
taken place along the west coast of South America during
earthquakes have been mentioned in Chap. 111. But
Darwin obtained evidence that, besides these sudden
paroxysmal movements, there had been a slow and imper¬
ceptible rise of the land, and that the elevation had pro¬
bably been greater inland than on the coast. Eaised
beaches occur along both sides of the continent, and form
a succession of terraces, which are in some places very
CHAP. Y.] ELEVATION AND DEPRESSION OF LAND. 71
conspicuous. Thus, " at Coquimbo, five narrow, gently-
sloping, fringe-like terraces rise one behind the other, and,
where best developed, are formed by shingle. They front
the bay, and sweep up both sides of the valley. At G-uasco,
north of Coquimbo, the phenomenon is displayed on a
much grander scale, so as to strike with surprise even some
of the inhabitants. The terraces are there much broader,
and may be called plains. In some parts there are six of
Fig. 19. Section through the Brighton Cliffs.
a. Elephant hed. o, hed of sand.
b, anciênt shingle. c c, chalk.
them, but generally only five : they run up the valley for
thirty-seven miles from the coast." ^
England.—Raised beaches have been found at several
places along the southern and western coasts—as, for in¬
stance, in Cornwall," at Portland," Selsey, and Brighton.
The following account of the Brighton beach is taken from
Mantell's " Medals of Creation" ; and fig. 19 shows the
arrangement of the beds forming the cliffs, as seen in a
ravine between Kemptown and Rottingdean in 1836.
1 " Voyage of the Beagle," Darwin, 1860, p. 343. Many of these
beaches contain sea-shells, as already mentioned, p. 68.
' ® See De la Beche, "Geology of Devon and Cornwall," and
Ussher, " Geol. Mag.," 1879, p. 166.
" See Prestwich, "Quart. Journ. Geol. Soc.," xxvüi. p. 38;
xxxi. p. 29.
72
DYNAMICAL GEOLOGY.
[sec. i.
At the base of the cliffs, and hounding the modern
beach, there is a low wall or terrace of chalk, and upon
this rests a bed of sand (o, in fig. 19), of irregular thick¬
ness and variable extent. From this sand marine shells
and the jaw of a whale have been obtained. Upon this
sand is a bed of loose shingle similar to that of the present
beach, though the pebbles of which it consists are not all
flints. In this ancient shingle the teeth and bones of
various extinct animals have been discovered.
The rest of the cliff above the shingle bed consists of a
mass of chalk rubble and loam, obscurely stratified, and
varying in thickness from 50 to 120 feet. This is a ter-
restial deposit, and was named the "Elephant bed" by
Dr. Man tell, on account of its containing so many teeth
and bones of the extinct elephant, known as the mammoth,
together with the remains of other animals, such as the
rhinoceros, horse, deer, and oxen.
These facts demonstrate the following series of changes
in the relative level of land and sea :—
1. The chalk terrace on which the sand and shingle rest
was at the sea-level for a long period, and a beach was
formed at the base of the chalk cliffs then existing.
2. The land was elevated, probably, to a higher level
than that at which it now stands, and the Elephant bed
was accumulated in an old hollow or valley.
3. The present cliffs, with the modern beach at their
base, have been formed by the action of the sea, which
will be described in a future chapter.
Scotland.—Well-marked terraces with raised beaches
have been traced at intervals along the Scottish coasts, four
or five sometimes occurring one above the other, at heights
of 25, 40, 60, 75, and 100 feet above the present high-
water mark.' The lowest of these is thus described by
Professor A. Geikie :—" For many miles on both the eastern
and western sides of Scotland a flat platform, but slightly
raised above the sea, winds along the coast, bounded on
the outer edge by the line of high water, on the inner by
a more or less precipitous bank of clay or rock. Most of
' " Good Words," 1868, p. 62. See also " The Scenery and
Geology of Scotland," second edition, 1887, p. 38.
CHAP. V.J ELEVATION AND DEPRESSION OF LAND. 73
the seaport towns are built upon this level terrace, and
it furnishes an admirable route for roads and railways
which skirt the shore. It consists of stratified sand and
gravel, sometimes full of sea-shells, and arranged after
the same fashion in which similar materials are now being
deposited upon the present beach. The platform is in
truth an old sea-beach, and marks a time when the land
was 20 or 80 feet lower than it is to-day, and when the
sea broke against the line of steep slope or cliff which
now rises as a green leafy bank along the inner edge of
the beach." The platform is covered with meadows and
cornfields which run up to the base of the cliffs. (Pig. 17.)
5. Testimony of Old Sea-Caves.—The formation of caves
by the action of the sea-waves beating against a line of
cliffs will be described in a future chapter. The caves
are excavated at the foot of the cliffs, and consequently, if
the coast is afterwards elevated, these caves will remain as
evidence of the level at which the land formerly stood.
Excellent examples of such old cliffs and caves occur at
various heights along the rocky coasts of Scotland up to a
level of 100 feet above the sea, and are often associated
with the raised beaches (as in fig. 20).
Along the coast of Can tyre there are numerous caves at
the foot of the cliffs which border the 25 foot beach, which
is thus described by Professor Hull : ^—" All along the
coast the ancient sea-margin may be traced by a line of
Fig. 20.
a, raised beawjh.
b, cave.
c, cliff.
s s, high-water mark.
' " Geol. Mag.," vol. iii. p. 6.
74
DYNAMICAL GEOLOGY.
[sec. i.
cliffs of various degrees of steepness according to the
nature of the rock. . . . Prom the base of this cliff a
slightly shelving terrace extends to the present sea-margin,
on which' most of the villages are built. . . . The caves
whch are found at intervals all round the coast, and which
form a range of natural rock-hewn compartments at a level
of 10 to 30 feet above the present tidal limits, are perhaps
the most convincing of all the various evidences of ancient
sea-action." Some of these caves near Campbelton and
Neill are specially described, and extend into the cliff for
a distance of 120 feet.
Proofs of Depression.
It has already been remarked that the proofs of the
submergence of land are not generally so obvious as
those of its elevation ; so that to a careless observer eleva¬
tion will appear to have been more general than depres¬
sion in most parts of the earth. In some cases, however,
there is direct evidence in the actual submergence of build¬
ings and former land-surfaces ; and in other cases the fact
of depression can be as certainly deduced from the position
of the river valleys in relation to the present level of the
sea.
1. Testimony of Human Erections.—Where the land
has been sinking for a long time, the ruins of ancient
towns and buildings are sometimes visible beneath the sea ;
as, for instance, at the east end of Candia, mentioned on
p. 66. It is necessary, however, to point out that the
mere encroachment of the sea upon the land cannot be
accepted as proof of subsidence, because, as will be ex¬
plained in a future chapter, the waves can cut back a
coast line, and cause the successive disappearance of fields,
houses, and villages, without being aided by any movement
of depression. It is requisite, therefore, that the relation
of the buildings to the sea should be such as to demonstrate
that an actual change of level has taken place.
This is the case on the western coast of Greenland,
which was surveyed by .Captain Graah and Dr. Pingel between
the years 1823 and 1832. They arrived at the conclusion
CHAP. V.J ELEVATION AND DEPRESSION OF LAND. 75
that the land had been sinking for the last four centuries
between Igaliko, in 60° 43' N. lat., and Disco in 69°, a space
of 600 miles. " Ancient buildings on low rocky islands
have been gradually submerged, and experience has taught
the aboriginal Greenlander never to build his hut near the
water's edge. In one case the Moravian settlers have been
obliged more than once to move inland the poles upon
which their large boats were set, and the old poles still re¬
main beneath the water as silent witnesses of the change." ^
Again, in Scania, the most southerly part of Sweden, there
is similar evidence that the land is sinking. In many of
the seaport towns some of the streets are below the level
of high-water mark, and ancient streets have been found
at a still lower level. Thus, at Malmö, one of the present
streets is overflowed by the waters of the Baltic when the
wind is high, and excavations made some years ago dis¬
closed an ancient street at a depth of 8 feet below the
present one. There is a large stone at Trelleborg, the
distance between.which and the sea-margin was measured
by Linnaeus in 1749, and in 1836 it was found to be 100
feet nearer the water's edge than it was eighty-seven years
before.
3. Testimony of Submerged Forests.—When a tract of
land is depressed beneath the sea, its superficial layer is
generally removed by the action of the waves ; but under
favourable circumstances, in sheltered bays and estuaries,
or by the formation of sand dunes, the old terrestrial
surface is sometimes preserved, and the stumps of old
trees are often seen on the shore at and below present low-
water mark. De la Beche thus describes the remains of
such an old land surface exposed along the south-west
coast of England." " Bound the shores of Devon, Corn¬
wall, and West Somerset, a vegetable accumulation, consist¬
ing of plants of the same species as those which now grow
freely on the adjoining land, is frequently discovered, occur¬
ring as a bed at the mouths of valleys, at the bottoms of
sheltered bays, and in front of and under low tracts of
land, the seaward side of which dips beneath the present
level of the sea."
' Lyell, "Principles of Geology," ii. p. 196.
^ " Geological Report on Devon and Cornwall," p. 420.
76
dynamical geology.
[sec. i.
Similar beds of peaty matter occur at numerous points
all round the coasts of Great Britain and Ireland, proving
that the last movement to which these islands have been
subjected, was one of depression. Two instances may be
adduced :—
1. In the harbour of Holyhead a bed of peat 3 feet thick,
with the stumps and roots of trees, is exposed at low water,
and stretches upward to a slight elevation above the sea,
where the excavations made for the railway in 1849, showed
that it was covered by stiff blue clay, and brought to light
two perfect heads of the extinct elephant known as the
Mammoth.
2. A submerged forest of large exent exists off the north
coast of Norfolk from Hunstanton to Brancaster Bay.
The portion exposed at low-water, north of Hunstanton
cliff end, is about a mile and a half from high-water mark,
and consists of a thick bed of black peaty matter, composed
of small twigs, branches, leaves, and other vegetable re¬
mains matted together, and enclosing the trunks, stumps,
and roots of large timber trees. The forest here occupies
an area of at least 500 or 600 acres, and among the roots
are occasionally found the remains of deer and oxen,
proving that these animals once roamed through the forest.
Possibly also they may have been hunted here by the early
inhabitants of Britain, for a flint-celt or axe was found em¬
bedded by its cutting-edge for a depth of an inch and a
half in the trunk of one of the trees. A similar forest-bed
is found along parts of the Lincolnshire coast, and is over¬
laid by the recent clays of the marshland.'
3. Testimony of Buried Valleys.—That rivers make their
own valleys will be proved in a subsequent chapter, but
their excavating power generally ceases long before they
reach the sea, and in no case can a river deepen its channel
more than a few feet below the low-tide level. When,
therefore, borings disclose the existence of valleys, the
rocky bottoms of which are 50, 70, or 100 feet below the
sea-level, and which are filled up with deposits of gravel,
sand, and clay, we may conclude that they are parts of an
^ For other examples of submerged forests, see "Geol. Mag.,"
vol. vii. p. 164, and Dec. 2, vol. iii. p. 491. " Quart. Journ. Geol.
Soc.," xxxiv. p. 447.
CHAP. V.J ELEVATION AND DEPRESSION OF LAND. 77
old land surface whicli has sunk beneath the sea. When
such valleys were formed the land must have stood at a
higher level, the sea must have been further ofE, and the
valley floor must have been above its level ; consequently, if
the bottom of the valley is now 100 feet below the sea-
level, it is proof that there has been a subsidence of at least
that extent.
Such buried valleys are not uncommon in the British
islands ; for instance, the rivers Yare and Waveney, in Nor¬
folk, flow through flat alluvial tracts for many miles be¬
fore they reach the sea, the width of the valleys being great
in proportion to their apparent depth ; though the real
depth of the original valley is in accordance with this
width. In the valley of the Tare, at Wroxham Bridge,
Horizontal scale, 2 inches to a mile. M, present valley ; o, ancient
valley filled with drift ; w, site of Winnes ; E, site of Kuncorn ;
s s, sea-level.
there is as much as 70 feet of detrital matter between the
present alluvial level and the chalk which forms the bottom
of the actual valley.
Borings at the mouth of the river Tees prove that the
bottom of the ancient valley is about 200 feet below
Ordnance datum, or mean sea-level; and a boring near the
mouth of the Tyne was carried to a depth of 124 feet with¬
out reaching the rock-bed of the valley.
Again, borings at Widnes, on the north side of the
estuary of the Mersey, have shown that there is a buried
valley nearly a mile in breadth and with an extreme depth
of 163 feet from the present surface, and 141 feet below
Ordnance datum. Fig. 21, which is copied from that drawn
by Mr. Mellard Beade, shows the comparative size of the
present and the ancient valley of the Mersey between Eun-
com and Widnes.
Similar old valleys and river channels have been dis-
78
DYNAMICAL GEOLOGY.
[sec. i.
covered in Scotland, so completely buried by subsequent
deposits that there is no indication of their presence on the
surface ; in some cases the sites of these buried valleys are
covered by low hills or mounds more than 100 feet high,
so that the courses of the ancient rivers must have been
very different from the present streams.
One of these old river channels has been traced from
Kilsith in Stirlingshire to Grangemouth on the Firth of
Forth. The surface of the ground at Kilsith is now 160
feet above the sea, and the old valley bottom lies at a
depth of 120 feet, or 40 feet above the sea; thence it slopes
gradually eastward till at Grangemouth the old channel is
260 feet below the sea-level.'
At the time, therefore, when this channel was occupied by
the river which made it, the land must have been at least
260 feet higher out of the sea than it is now, an elevation
which would be sufficieut to unite the British islands to
the continent of Europe-
4. Testimony of Fiords.—A fiord is a long narrow inlet
of the sea which does not terminate abruptly, but is the
continuation of an inland glen or valley. It is, in fact, part
of a depressed valley which is filled with sea-water in¬
stead of being choked up with sand and mud, as in the case
of the English valleys. The term fiord is a Norwegian name
for such narrow inlets, and is the same as our firth ; it is the
multitude of these firths or fiords which cause the outline
of the Scottish and Norwegian coasts to present such a
peculiarly irregular and indented appearance. All the
larger glens and valleys of western Scotland terminate in
fiords, or lochs as they are there called, which are simply
the submerged prolongations of the glens ; their existence
is a proof that the whole country once stood at a higher
level than it does now, and that it has not yet been raised
again to the level it occupied when the glens and channels
were formed. In the same way the fiords of the Norway
coast prove that a depression took place before the
present rise of the coimtry, and that the land has not
yet been re-elevated to the level at which it previously
stood, when these ancient valleys were first excavated.
' J. Croll, "Trans. Geol. Soc. Edin.," vol. i. p. 330.
CHAP. V.J ELEVATION AND DEPRESSION OF LAND. 79
5. Coral Beefs.—Certain forms of coral reefs are by some
geologists regarded as evidence of subsidence. The growth
and formation of such reefs will be described in a future
chapter (xvi.), and the view that barrier-reefs and atolls
can only be found in a sinking area will be discussed.
Darwin, Dana, Jukes, and other observers have maintained
this view, but it is.opposed by most recent writers on the
subject. Eaised coral-reefs are, of course, as undisputable
proofs of upheaval as raised beaches are.
General Conclusion.—Combining all the evidence
which has been presented to the reader in this and the
preceding chapter, we arrive at the conclusion that the
earth's crust, instead of being a rigid, immovable mass, as
was formerly supposed, is, and always has been, utterly
unstable. It is probable, indeed, that no part of the land
remains stationary for any long period of time, geologically
speaking ; but is eventually either slowly depressed, or as
slowly upraised to a stiU higher elevation above the sea.
It is certain, at any rate, that the form of the great con¬
tinents has been continually altered, parts being elevated,
and parts depressed, so that every portion has in its turn
been brought beneath the level of the sea. It is certain,
also, that every country is now dry land only because it
has been upraised from beneath the neighbouring sea.
This great truth will be still further demonstrated in sub¬
sequent chapters.
SECTION II.—Changes produced by the Agencies
which operate orí the Surface of the Earth's Crust.
IN civilized countries and temperate climates, and espe¬
cially in regions like our own, where volcanoes are un¬
known, where no lofty mountain ranges exist, and where
all natural phenomena, such as rainfall, rivers, hills, and
valleys, are on a comparatively small scale, people are apt
to underrate the amount of change which is really taking
place all over the surface of the earth, but which is much
more apparent and rapid in some regions than in others.
Moreover, those parts of our own cormtry where natural
agencies of change are most active are, as a rule, the most
thinly populated ; men do not congregate on high
mountains or on rocky coasts, but build their towns in
the lowlands and fertile districts, where perhaps only an
occasional flood or storm serves to remind them that the
forces of nature are not always gentle and regular in their
action.
Lastly, the efforts of the human race are continually
directed to the control of these natural forces ; ditches are
dug to carry off the rain that falls on the land ; rivers
are embanked and made to keep within definite courses ;
walls, groynes, and breakwaters are constructed to stop the
sea from eating away the coast, and the excessive action
of every natural agency is prevented as far as possible.
It is not surprising, therefore, that those who live in a
lowland country, and who have never travelled far from
the homes in which they were born, should find it difficult
to realize the work that may be done, and the results that
are actually accomplished, by the uncontrolled forces of
nature in other parts of the world. They see around them
in old age the same fields, the same slopes, and the same
watercourses which they knew when they were children,
CHANGES ON SURFACE OF EARTH'S CRUST. 81
and they are apt to think that things had always been as
they had always seen them.
In these days, however, people travel very much more
than their ancestors did, and many thus become aware
that in uncultivated districts, on exposed coasts, on moor¬
lands and mountains, and in regions that are exposed to
the extremes of heat or cold, operations are in progress
which often cause a large amount of change even within
the duration of a human lifetime. In other countries, too,
the traveller may view the delta of a large river, he may
visit a coral island, he may perchance experience or see the
effects of an earthquake shock, or he may find sea-shells
of recent species bleaching on slopes that are now several
himdred feet above the sea. Still, it does not often fall to
the lot of one man to witness all these things, as well as
the many other evidences of change that might be men¬
tioned ; and many a traveller has returned home without
comprehending the part which such changes have played
in the economy of nature.
There are, moreover, certain processes of change which
the physical conditions of man's existence make it difl&cult
for him to study, even when he specially desires to do so.
Those who live by the seaside are familiar with some of
the alterations effected by waves and currents on its
shores, but they cannot see below the surface of the open
sea and watch the drifting of sand into banks, or the slow
accumulation of mud and ooze in the deeper water. It
requires the dredge and sounding apparatus to give us any
information about the deposits which are being formed at
the bottom of our seas and lakes. Consequently, as Sir
Charles Lyell remarks,' "It is not surprising that we
estimate very imperfectly the result of operations thus in¬
visible to us ; and that when analogous results of former
epochs are presented to our inspection, we cannot imme¬
diately recognize the analogy. He who has observed the
quarrying of stone from a rock, and has seen it shipped
for some distant port, and then endeavours to conceive
what kind of edifice will be raised with the materials, is in
the same predicament as a geologist, who, while he is con-
' " Principles of Geology," tenth edition, vol. i. p. 99.
G
82
DYNAMICAL GEOLOGY.
[SKC. II.
fined to the land, sees the decomposition of rocks and the
transport of matter by rivers to the sea, and then endea¬
vours to picture to himself the new strata which nature is
building beneath the waters."
In the following chapters we shall endeavour to assist
the reader to form some conception of the effects which
are produced by these processes of destruction and con¬
struction, by bringing to his notice such examples of their
action as have been described by experienced observers in
various parts of the world.
It is assumed that the student has read some ele¬
mentary treatise on Physical G-eography, and has thereby
acquired some knowledge of common atmospheric pheno¬
mena, and of the general physical features of the globe.
He will then be prepared to follow the more detailed
descriptions of the important effects which heat and cold,
rain and frost, running water and moving ice, produce
upon all surfaces exposed to their action. Some of these
agents are chiefly engaged in breaking up and wearing
down the rock-masses, others are more concerned in trans¬
porting and re-arranging the detritus, but the ultimate
result to which all contribute is the re-construction of
these materials and their deposition as new formations in
the beds of rivers, lakes, and seas.
In considering the operations of these agencies, there¬
fore, it will be desirable to arrange our descriptions in two
divisions, confining our attention in the first part to their
destructive and dispersive effects, and leaving to the second
division the consideration of what may be termed their col¬
lective and reconstructive effects.
The processes of disintegration and erosion with which
we are to deal in this and the following chapters may be
thus classified :—
Agents. Action.
I. Terrestrial Ageaeies. j
II. Flurialil, Ageneies. | «S» ! ' ! ! TÂ J"
III. Marine Agencies. | | Mechanical.
A. Processes of Disintegration.
CHAPTEE VI.
I. TEBRESTEIAL AGENCIES.
IT is common matter of observation that aU rock-sur¬
faces which have been exposed to the action of the
weather for any length of time show a tendency to decay
and crumble away. The agencies concerned in producing
this weathering, or disintegration, are several ; they act both
chemically and mechanically, but they are all so intimately
connected in their operations, that any given case of dis¬
integration can generally be shown to be the result of the
conjoined action of two or more of them.
They may, however, be arranged as follows, and we shall en¬
deavour, as far as possible, to consider their separate effects.
Mode of action.
Chemical and
Mechanical.
Mechanical.
Mechanical.
Mechanical.
Chemical and
Mechanical.
The mechanical effects of these natural agents can be
easily understood, but to comprehend their chemical
action, the student must have some knowledge of the
rudiments of chemical science. These are now so fre¬
quently taught in schools, that we assume the reader has
a suf5.cient knowledge of chemical elements and their com¬
pounds to understand the explanation of the chemical
Atmospheric.
Underground =
Agents.
Eain . . . .
Wind . . .
Heat and cold.
Erost. . . .
Percolating water
84
DYNAMICAL GEOLOGY.
[sec. ii.
action of water ; but if he has not this knowledge, he will
find a brief notice of such compounds, and of the principal
rock-making minerals, in the second part of this book.
Atmospheric Agents.
Rain, and what becomes of it.—All the rain which
falls upon the surface of the land is disposed of in one of
three ways : 1. Some of it is immediately returned again to
the atmosphere by evaporation. 2. Some of it trickles down
slopes, and collects into rills and streams, which flow over
the surface of the land. 3. Some of it sinks into the
groimd, and percolates through cracks and joints of the
rocks. The rain-water which flows over the surface operates
as a mechanical agent, and carries off with it particles of
the soil or rock over which it passes in mechanical sus¬
pension. The rain-water which sinks into the ground acts
chemically, it dissolves certain portions of the rocks, through
which it percolates, and carries them away in chemical
solution.
Some of this percolating water is thrown out again to
the surface in the form of springs, but the remainder
penetrates still further into the substance of the earth,
where, with the aid of heat and pressure, it effects great
changes in the chemical constitution of the more deeply-
seated rocks, and is largely concerned in producing the
phenomena of hot springs.
The Chemical Action of Rain.—Carbonic and Humus
Acids.—Before proceeding to describe the chemical effects
of rain-water on the earth, a few words of explanation are
necessary, in order that the source of its chemical energy
may be properly understood. Pure rain-water would have
little effect upon any rock-substance except common salt ;
its rotting and dissolving powers are mainly due to its
capacity for taking up and absorbing certain active agents,
viz., carbonic acid (CO,), the humus acids, and oxygen.
Carbonic acid gas exists everywhere in the atmosphere
surrounding the earth, though the quantity contained in
the air at different places varies considerably. All animals
exhale carbonic acid from their respiratory organs, and aU
CHAP. VI.J TEREESTKIAL AGENCIES.
85
living plants decompose carbonic acid, absorbing the car¬
bon and setting free tbe oxygen. Dead vegetable matter,
by its decomposition and oxidation, gives rise to a set of
organic acids, of which the most important are crenic,
humic, and ulmic acids, often called collectively the "humus
acids." The nature and action of these acids are not yet
fully understood, but they are known to have a remarkable
power of dissolving many of the mineral substances which
occur in rocks. They appear to combine readily with cer¬
tain metals (such as iron), forming soluble compounds,
which are carried away in water and often re-precipitated
in other localities. They decompose certain silicates, and
will even dissolve silica, the presence of minute quantities
of silica in all streams being probably due to their agency.
All these acids, as well as carbonic acid, are readily
absorbed or dissolved by water ; and, as a matter of fact,
all water on the earth's surface contains a greater or less
amount of them. Eain takes up carbonic acid from the
air through which it falls, and obtains still more, together
with humus acids, from the soil through which it per¬
colates. Water thus charged with acids is everywhere
soaking into the earth, and it exercises a powerful disin¬
tegrating action on the rocks throqgh which it percolates.
Some of their constituents are chemically attacked, and
carried away in solution ; other particles become loosened
in the process, and are eventually removed by the me¬
chanical action of the rain-streams.
This gradual decay and disintegration goes on wherever
the surface of a rock is kept continually wet with rain and
moisture. Plants increase the chemical action by enabling
the moisture to lodge and remain for a longer time on the
spot, but they retard the mechanical action, and prevent
the loosened particles from being carried away, so that
these accumulate and form a soil.
The substances soluble in acidified water are :—
Chlorides.
More Carbonates,
soluble. Sulphates.
Nitrates.
/"Most Oxides.
Less Silicates of
soluble. the alka¬
lies.'
' The commonest alkalies are sodium and potassium.
86
dynamical geology.
[sec. ii.
The less soluble substances are, however, more soluble
m water which has already dissolved alkaline compounds ;
thus, if rain-water in soaking through rocks near the
surface takes up much carbonate of sodium or potassium,
it is able to dissolve more of any alkaline silicates it may
meet with in its further course through the rocks below.
Moreover, if the action of the alkaline solution is aided by
that of a high temperature, as is often the case in volcanic
districts, large quantities of silica or of silicate may be dis¬
solved in the warm alkaline water.
But, besides these acids, all natural waters contain
another active agent in the shape of oxygen gas. This
readily combines with metals, and with all oxides which
can absorb more oxygen, and by this oxidation different
mineral compounds are formed. The rusting of an iron
nail is a familiar example of this action, and, similarly,
any protosalt of iron which may occur in a rock will be
oxidized, and a rusty brown colour will generally be
imparted to the decomposed and disintegrated mass.
Action of Bain on Calcareous Bocks.—Carbonate of
lime enters into the composition of many rocks; some,
indeed, such as chalk and other limestones, being almost
entirely composed of ik Since the material of such rocks
is readily soluble in carbonated water, it is evident that
they will show signs of rapid decay and disintegration.
A limestone which is nearly pure carbonate of lime, is so
completely dissolved that little remains to form a soil,
and that little is quickly swept off the surface by rain,
so that districts composed of such limestone are often
bare and barren, except in the hollows and valleys where
soil has accumulated.
Excellent instances of the chemical solution of lime¬
stone surfaces occur in certain parts of North Lanca¬
shire and Westmoreland. In these counties limestone
often forms flat-topped hills and "fells," and where the
rock surface is bare of soil it becomes worn and fretted
out into a network of open clefts and fissures, a foot or
two wide at the top, but gradually narrowing downward
to a mere crack or rift at a depth of four or five feet.
Protected by the sides of the fissures from the winter
winds, and finding foothold in the soil which fills the
CHAP. VI.J
TERRESTEIAL AGENCIES.
87
crevices, grow numerous ferus and other plants, while
mosses and lichens adorn those surfaces which catch the
greatest quantity of moisture, so that a peep into one of
these crevasses discloses a natural conservatory where the
luxuriant growth of vegetation is in strong contrast to the
hare greyness of the upper surface of the rock. The open
clefts follow the course of the natural cracks or joints in
Fig. 22. A Slab or Surface of weathered and eroded Limestone.
the limestone, and are clearly due to the widening of these
cracks by the chemical action of the rain. The annual
rainfall of such localities is often large, but all the rain
that falls on the fell finds its way into the cracks and
crevices, and it is only where the surface is sloping that it
can exercise any mechanical action. The water is arrested
by the plants which flourish in the fissures, the mosses and
ferns especially acting as so many sponges to retain the
water. Then as it passes down through the roots and
88
DYNAMICAL GEOLOGY.
[SKC. II.
decaying vegetable fibres, it takes up the acids which
these evolve, and becomes an active chemical agent for
widening and deepening each crevice and cranny.
In some places it is even possible to estimate the actual
amount of rock which has been removed in solution since
the rock surface has been exposed to the present atmo¬
spheric and climatal conditions. Professor Hughes has re¬
cently described some cases of " pedestal boulders," which
are boulders of some rock foreign to the locahty, standing
on low flat pedestals of the local limestone ; the surface of
the pedestals or tables is not like that of the surrounding
rock, but is smooth and often striated by the passage of
ice (a process which will be explained in a future chapter) :
we are only now concerned with the fact that the surface
of the fell was a smooth one at the time when the boulder
came to rest there, and with the further fact that the
presence of the boulder has protected a portion of this
surface from the wasting action of the rain, while the
surrounding parts, to a depth of from twelve to eighteen
inches, have been dissolved and carried away. Professor
Hughes comments on the influence exercised by vegetation
on the mode of weathering, and says : " We see crevices
where there can never be any mechanical erosion of the
rock opened out into great chasms as lichen, moss, ferns,
and grass successively get foothold in it ; and roimd the
margin of the great hmestone tables on which the boulders
rest we see the manner in which the rock is eaten back
as the vegetation encroaches along the lines of weakness,
furnishing more acid and holding more damp." '
The phenomena of the open clefts and chasms are pre¬
sented on a larger scale on certain limestone plateaux in
the Alps, in Bohemia, and in Saxony, which are known as
1carren-felder. The whole surface of these plateaux is
worn into a labyrinth of deep and narrow gutters (karren),
which have been formed mainly by solution, but partly by
mechanical erosion ; these ar^ often 6 or 8 feet and occa¬
sionally from 10 to 20 feet deep.
Limestones which contain an admixture of sand or
siliceous matter are converted into the substance known
* "Quart. Journ. Geol. Soc." vol. xlii. p. 533.
CHAP. VI.] TERRE.STRIAL AGENCIES.
89
as rottenstone ; tlie calcareous portion being eaten out and
carried away in solution, while the insoluble particles are
left to form a rotten and friable rock. Impure limestones
containing argillaceous (or clayey) matter are similarly
disintegrated, the insoluble residue remaining, and crumb¬
ling down to form a soil. Such a residual soil, thereforg,
gives us a rough measure of the amount of rock removed
by solution.
By the double process of dissolution and oxidation,
calcareous sandstones are often reduced to the condition
of a loose brown sand, or friable sandstone. Sands that
were originally full of calcareous shells are frequently con¬
verted into a ferruginous sand, entirely devoid of organic
remains. Thus, in the east of England there are certain
shelly sands, called crag, the surface beds of which have
been so altered by this process that they were long be¬
lieved to be a distinct formation. The line of separation
between the altered and unaltered portions is always
clearly defined, but very irregular, because the depth of
the former varies much according to local conditions. ^
The same phenomena have been observed in Belgium,
where calcareous sands have been decalcified and oxidized
to a large extent, and their true relations to the under¬
lying beds were quite misunderstood until this fact was
recognized by M. Vanden Broeck.'
The decalcifying action of surface waters is excellently
shown in the case of certain sands and gravels which occur
in the eastern counties of England, and consist partly of
flint sand and partly of small chalk pebbles, and what may
be called " chalk sand." Pits dug in these deposits show
a sharp contrast of colours and materials ; the upper part,
beneath the surface soil, to a variable depth, is a brown
sandy gravel, from which all the chalk has been dissolved,
while the material below is pale yellow, or nearly white,
from the quantity of chalk it contains (see fig. 23).
2. Action on Ferruginous Rocks.—The humus acids
formed by the decay of vegetation, and dissolved in rain¬
water, play an important part in the dissolution of
^ See Whitaker, " Quart. Journ. Geol. See." vol. xxxiii. p. 122.
' See " Geol. Mag." ser. 2, vol. viii. p. 279.
90
dynamical geology.
[sec. ii.
fernigiuous minerals, and in the solution, transference,
and re-deposition of iron.
Most sands and sandstones are more or less coloured by
iron, either in the form of the red ferric oxide, or of the
eompound green silicate known as glauconite, and where
such sands form a tract—heath, moor, or high forest-coYcred
ground—through which surface waters can freely percolate,
many interesting chemical processes are in action.
In the case of ordinary yellow-tinted sands the crenic
acid formed in the surface soil takes up the iron into
solution as a ferrous salt, after reducing the peroxide to
the protoxide, which forms the base of a crenic salt. Such
salts are carried down in solution, and in this way the
upper part of the sands is sometimes decolorized, leaving
Fig. 23. Decalcification of Sand and GraveL
a, Unaltered chalky gravel. b, Bro-«Ti sandy gravel.
a white sand, which is prized for glass-making and other
purposes. If the percolating water is thrown out as springs,
the iron is precipitated by similar chemical action, as the
water is oxygenated, in the form of hydrous ferric oxide or
limonite, and appears as a scum or soft flocculent deposit
on the banks of the issuing stream. In dry seasons, when
the water-level is lowered, and the sands are in places com¬
pletely dried up, air makes its way down at ordinary
atmospheric pressure, and the oxygen acts on the ferrous
compounds, causing their precipitation, partly as Hmonite,
and partly as basic crenate of iron.'
Glauconitic sands are extensively affected by humus
acids, the glauconite being decomposed, and converted into
limonite, while silica is set free ; so that sometimes a great
' I am indebted to Dr. A. Irving for the above information.
CHAP. Tl.]
TERBESTKIAL AGENCIES.
91
depth of the sand is changed from a green sand into a
yellowish or rusty-brown sand. This result is particularly
conspicuous where the country is or has been wooded, as in
the case of the large tracts of greensand on the borders of
Wilts and Somerset, which was formerly part of Selwood
Forest, the decay of the forest litter furnishing the acids.
These sands also contain, in some places, lumps of sand
cemented by crystalline silica, which may, perhaps, have
been formed by re-deposition of the silica set free in the
decomposition of the glauconite.
Lastly, volcanic rocks, as basalt, often yield a deep rusty
brown soil, the formation of which is doubtless largely due
to the action of humus acids on the augite and hornblende
they contain, breaking up these silicates, and reducing the
iron to limonite. This is especially noticeable in tropical
countries, where the growth and decay of vegetation is
more rapid, and is probably the chief agency concerned in
the production of laterite, which will be noticed in another
chapter.
3. Action of Rain on Felspathic Rocks.—It has been
already stated that it is not only the rocks containing car¬
bonate of hme which are weathered and worn away by the
action of acidified water. Those containing minerals com¬
posed of the silicates of lime, soda, or potash, yield almost
as readily. Granite affords an instance of this, for the
felspar which it contains consists of the combined silicates
of alumina, potash, and soda. Rain-water falhng upon
granite decomposes the felspar, removing the sihcates of
potash and soda, either in the form of a soluble silicate, or
by converting them into carbonates, but leaving the silicate
of alumina and the other constituents (quartz and mica),
which are very insoluble.
By this process the substance of the rock is sometimes
so loosened and disintegrated that it can be dug out with
a spade. On slopes where the rain-water acts mechani¬
cally as well as chemically, the materials of the rock are of
course removed as fast as they are loosened, being carried
away and deposited elsewhere, partly as clay or kaolin
(sihcate of alumina) and partly as sand (particles of
quartz). The large deposits of kaolin (or china clay) in
Cornwall illustrate the enormous destruction of granite
92
DYNAMICAL GEOLOGY.
[sec. 11.
which has there taken place. Orthoclase felspar, which is
the variety usually present in granite, is a trisilicate of
alumina and potash, with the following normal com¬
position :—
Silica ....... 64"6
Alumina ...... 18"5
Potash ....... 16'9
Kaolin, when quite pure, is regarded as a hydrated bi-
silicate of alumina, with the theoretical composition of—
Silica ....... 46*6
Alumina ...... 39"5
Water 13"9
Hence we may infer that the compound trisilicate is de¬
composed by the action of rain-water ; that all the potash,
and part of the silica is removed in solution ; while the
alnmina and remaining sihca have re-united, and taken up
water to form a fresh mineral snbstance. The change may
be chemically expressed as follows :—
Orthoclase and water = kaohn, potash, and silica.
The extent of the chemical action is, however, better de¬
monstrated in districts where the decomposed granite is
not carried away so rapidly, but is left as a loose crumbling
mass. A good instance of this is given by Mr. W. Kings-
mill in describing the granite of the moimtains near Canton
in China. He says :—" This granite, wherever it occurs,
has been deeply disintegrated, sometimes to a depth of 100
or 200 feet, whilst everywhere, imbedded in the soft,
yielding matrix, there occur nodules, of more quai4zose
character, which have resisted the effects of time and
chemical change. The original quartz veins of the granite,
broken into small fragments, still traverse the disintegrated
mass in all directions." ^
^ "Journal of the Geol. Soc. of Dublin," vol. x. p. 2.
CHAP. VI.]
TERRESTRIAL AGENCIES.
93
In Devon and Corawall granite is often thus decomposed
to depths of 80 or 40 feet, and where excavated presents
the appearance in fig. 24. The loose sandy part (6) is
shown to be simply the decomposed substance of (a) by the
detached lumps and cores of undecayed granite (c) which
are scattered here and there throughout the mass. The
veins of quartz, too, which occur in the solid granite, and
resist the chemical action of the rain-water, are continued
upwards through the disintegrated portion, and prove that
the whole originally formed one solid mass of rock. An
excellent example of this is to be seen in the Carclaze mine,
near St. Austell's, Cornwall.
V, Vein of quartz. s, Vegetable soil. c, Core of granite.
6, Decomposed granite. a, Solid granite.
Conversion of Calcareous Bocks into Ironstones.—There
is good reason to believe that some beds of ironstone, which
are now worked for iron, were originally limestones, which
have been converted into ferruginous rocks by the agency
of humic and crenic salts of iron. The Northampton iron¬
stone, for instance, is supposed to have been originally a
shelly calcareous rock, which has been replaced by carbo¬
nate of iron, and ultimately by the hydrated peroxide
(hmonite). When quarried, the surface portion of this
rock is a brown, porous, friable, earthy limonite; but
deeper down it is a hard, compact rock of a bluish-grey
colour, consisting mainly of carbonate of iron. Mr.
Hudleston considers it certain that this iron has been in¬
troduced, and that the entire mass was originally a sandy,
oolitic limestone, like the Dogger of Yorkshire, into which
94
DYNAMICAL GEOLOGY.
[sec. ii.
it can be traced, and in which the same fossil shells are
found.'
Mr. Hudleston did not see his way to accounting for the
iron ; but Dr. A. Irving believes it to have come in the
state of a crenic or humic salt of iron, formed in the over¬
lying soil, and carried down by water percolating through
the rock when it was a limestone. A complete reaction
then took place : the calcium (as the stronger base) took
up the strong acid of the iron-salt, leaving carbonic acid
available for union with the iron as ferrous carbonate
(FeCOj). But under such conditions this carbonate would
not remain ; by the alternate access of oxygenated rain¬
water, and of atmospheric oxygen in dry seasons, it would
be slowly broken up, the ferrous oxide being converted
into ferric oxide, and the carbonic acid going away in
solution. The process is illustrated by the condition of
the fossils ; shells originally made of carbonate of lime are
now converted into limomte.
In a similar manner portions of other limestones have
been converted into red haematite, or pure anhydrous per¬
oxide of iron, the iron having been derived from the soils
left by the red sandstones which once covered them.
Mechanical Effects of Rain.—Everyone is aware that
rain exercises a mechanical action in washing off the finer
particles of soil, or of chemically disintegrated rock : that
all rain falling upon the land and running over its surface
carries away a great quantity of such fine material, and
that the muddiness of all rills and streams after continuous
rain is a clear attestation of this fact. But most people
are not fully aware how greatly this action is intensified
when the rainfall is very heavy, and when it runs down
steep slopes ; the effects of a heavy rain-storm in a moun¬
tainous district should be witnessed before they can be
adequately realized, but it is no exaggeration to say that
the rüls of rain are capable of moving débris of a size
which would constitute gravel. In tropical countries the
wearing, excavating, and carrying powers of rain are still
greater, in proportion to the much greater volume of water
which descends in a given time.
' " Proc. Geol. Assoc." vol. xi. p. 122.
CHAP. VI.J TERRESTRIAL AGENCIES. 95
In temperate regions this wasting of the surface is more
gradual, but it is constantly going on, and though the
amount removed from any given area during one year is
hardly apparent, yet the course of years renders it appre¬
ciable, and cases sometimes occur in which the amount of
96
DYNAMICAL GEOLOGY.
[sec. ii.
detrition which has taken place can be actually measured.
An excellent illustration of the mechanical effects of rain is
furnished by the earth-pillars found in some of the valleys
of the Tyrol, and described by Sir Charles Lyell (see fig.
25). These are columns of indurated mud or clay, vary¬
ing in height from 20 to 100 feet, and usually capped by a
single stone or boulder. The history of their formation is
as foUows. The whole valley was once filled with the hard
mud or boulder clay, which is sufficiently solid to stand
with a vertical face like a hard rock, and contains nume¬
rous and often very large stones or boulders. Through
this material a moTintain torrent has excavated a deep
channel, leaving two platforms, one on each side of its
course (p p, in fig. 26). The heat of the sun causes the
Fig. 26. Section across the Valley of the Finsterbach, to show
how the Pillars are formed.
clay to crack in all directions, and these vertical cracks are
gradually, widened and deepened by the frequent rains
which fall in the valley. The rain-drops gather into rills
which course down the steep sides of the ravine, and its
banks would moulder away like those of other valleys,
were it not for the large stones which occur in the clay ;
these throw off the rain and protect the clay below from
its fretting action, while that around is washed away, so
that in time detached columns, or pyramidal pillars are
left, each capped by the protecting block which was the
initial cause of its development. Sometimes the capping-
stone falls off, and the column then terminates upward in
a point, or becomes separated into several smaller pillars
by the weathering out of other blocks within its mass,
which in their turn serve as new capping-stones.
CHAP. VI.]
TERRESTRIAL AGENCIES.
97
The desert country, called " the Bad Lands," in the
Washakie Basin, between Utah and Wyoming, U.S., affords
another good instance of rapid and extensive erosion by
rain. This district is a bare and barren plateau, consisting
of soft marls and sands lying in level layers, and termi¬
nating towards the south-east in a bold escarpment, which
is carved out into numerous blocks, turrets, columns, and
citadel-like masses of most peculiar and fantastic forms.
The effect is so architectural that from a little distance the
scarp-face presents the appearance of a great walled city,
with outlying bastions and buttresses. One of the most
remarkable of the detached masses is figured in the frontis¬
piece of the first volume of the "United States Exploration
of the Fortieth Parallel." This is an isolated column
springing from a level desert plain to a height of fully 60
feet ; its lower portion forms a kind of base or pedestal
9 or 10 feet high, above which the pillar narrows slightly,
and then rises vertically, till it terminates in a roughly-
truncated summit. This massive column looks like a
monument erected by the hand of man, and its general
shape bears no little resemblance to a lighthouse.
In describing these peculiar forms of erosion, Mr.
Clarence King, of the U.S. Survey, gives the following
explanation of the way in which they have been produced:'
" The small streams which cut down across the escarpment
from the interior of the plateau do the work of cutting the
front into detached blocks ; but the final forms of these
blocks are probably the efEect of rain and wind-storms.
The material is so excessively fine that, under the influence
of trickling water, it cuts down most easily in vertical lines.
A semi-detached block becomes quickly carved into spires
and domes, which soon crumble down to the level of the
plain. Outlying hills or huttes are carved away, leaving
narrow isolated spires, which finally disappear by the same
process of erosion. It seems probable that some of the
most interesting forms (like the column above described)
are brought out by a slightly harder stratum near the top
of the cliffs, which acts in a measure as a protector of
the softer materials, and prevents them from taking the
' " Exploration of the 40th Parallel," vol. i. p. 399.
E
98
DYNAMICAL GEOLOGY.
[sec. ii.
mound-forms that occur when the beds are of equal
hardness."
Good instances of the manner in which some rocks are
wasted away by the action of rain have come under the
notice of the writer while surveying a part of Lincolnshire,
where the ground consists of a soft, loose sandstone, rest¬
ing on a bed of stiff clay. The sandstone is easily dis¬
integrated by the action of rain and frost, and its surface
weathers into a loose sandy soil, the particles of which are
readily carried away by the mechanical action of the rain.
The borders of this district form sloping ground, trenched
by nnmerous gullies and valleys, worn by the riUs into
which the rain collects, and cut down near their termina¬
tions to the level of the underlying clay. Some tracts are
found where the once continuous sheet of the sandstone
has been reduced to the condition of low mounds or hills,
separated by wide interspaces, over which nothing but a
sandy soil remains to cover the snrface of the clay. These
interspaces are the sites of the hollows or gullies which
were worn in the sandstone, and were gradually deepened
and widened by the washing away of the sand, until the
surface of the underlying clay was bared. The remnants
of the sandstone are thus left as disconnected patches ; and
these remnants are still being reduced by the wasting
action of the rain, and are, as it were, gradually melting
away under its influence.
Sometimes a sandstone contains blocks or portions whicli
have been cemented into a hard stone by re-deposited silica ;
in such cases the blocks are left on the surface of the ground
after the removal of the softer portions. Such is the origin
of the large siliceous stones known as " Sarsen stones," or
grey wethers in Berkshire and Wiltshire ; these have been
derived from certain sands which once covered the chalk in
these and other counties, and they now lie scattered on the
chalk downs and in the valleys which lie between the hills.
In some places they are so numerous as to appear in the
distance like a flock of sheep, whence the name of grey
wethers.
Action of Wind.—Where the soil is dry, and loose
enough to be moved by the wind, such particles often
operate as powerful agents of detrition. Everyone is fami-
CHAP. VI.J TERKESTRIAL AGENCIES.
99
liar with the capacity of wind to blow away particles of dust
and sand from the surface of our streets ; and the erosive
power of a strong blast, charged with particles of sand, is
illustrated in the effects of the artificial sand-blast, by means
of which patterns are engraved upon glass. Natural sand¬
blasts produce similar effects upon the rocks with which
they come in contact ; but the results differ considerably,
according as the rocks, are of a nature to resist or to yield
to such mechanical action.
Very hard rocks, like basalt or compact limestone, when
exposed to the continual friction of blowing sand, are pitted
and polished in a peculiar manner. On the Fifeshire coast,
near Burntisland, for example, blocks of basalt can be seen
which have been polished in this manner by the sands of
the seashore. The author has also observed polished sur¬
faces on the sides and angles of the limestone rocks, near
the great pyramid in Egypt ; and the same effect has been
produced by wind-blown sand on the surface of many
Egyptian monuments.
Where, however, the rocks are soft, or their surface has
been disintegrated by the rotting action of rain-water, the
results are very different. Such rocks are not polished, but
are gradually abraded, and worn away by the friction of
the particles driven by the wind. The rapidity of this
action depends greatly on the power of the wind, and the
size of the particles which it carries ; but since the wind
cannot raise the larger particles of sand very high above
the surface of the ground, it foUows that the greatest
amount of erosion will take place at, or slightly above, that
surface. An isolated block of such a rock as sandstone, if
long exposed to sand-drift, will be worn away at the base
or undercut, until it presents the appearance of a gigantic
mushroom. Some of the celebrated " Brimham rocks," in
Yorkshire, exhibit this form very strikingly.
The " tors " and " logging stones," so frequent in granitic
districts, probably owe their peculiar form to the combined
action of wind and rain. Both granite and sandstone some¬
times form piles of rounded or sub-angular blocks, perched
one on the other, as if by artificial means ; and hence they
have sometimes been mistaken for Druidical remains.
When examined, however, by the light of geological know-
100
DYNAMICAL GEOLOGY.
[sec. ii.
ledge, it becomes clear that the apparent ruins are the
work of Nature's carving tools—wind, rain, and frost.
Fig. 27 represents such a pile weathered out of hard sand¬
stone near Lynton in Devonshire.
In arid climates, the agency of sand and dust-storms is
greatly intensified ; and Sir A. G-eikie believes that some of
the peculiar features of the sandy plateaus of Wyoming
and Utah, previously described, are largely due to the
action of wind-di-iven sand. " Blocks of sandstone or
limestone which have fallen from an adjacent clifiE are
attacked, chiefly at their base, by the stratum of drifting
sand, until by degrees they seem to stand on narrow pedes¬
tals. As these supports are reduced in diameter, the blocks
eventually tumble over, and a new basal erosion leads to a
renewal of the same stages of waste." ^
Action of Heat and Cold.—The mere vicissitudes of
heat and cold, without any assistance from freezing water,
sometimes exercise important effects, by means of the
alternate expansion and contraction which they cause.
This is especially noticeable in countries where the daily
range of temperature is very great, and is sometimes suffi¬
cient to crack and split off pieces of rock. Dr. Livingstone
mentions that, in the interior of Africa, he foimd rocks
which had their surfaces so rapidly cooled by radiation at
night that the contraction was sufficient to split the stone,
and to throw off sharp angular fragments, from a few
ounces to one or two hundred pounds in weight. The
following are his own words® :—
" Several of the mountain sides in this country (Gova)
are remarkably steep, and the loose blocks on them sharp
and angular, without a trace of weathering. For a time,
we considered the angularity of the loose fragments as
evidence that the continent was of comparatively recent
formation ; but we afterwards saw the operation actually
going on, by which the boulders are split into these sharp
fragments. The rocks are heated by the torrid sun during
the day to such an extent that the thermometer placed on
them rises to 137° in the sun. These heated surfaces,
' Geikie, " Textbook of Geolog," p. 320, and Gilbert in Wheeler's
" Report of the U. S. G. Surv. W. of 100th Meridian," üi. p 82.
' Livingstone's " Zambesi," p. 492.
Fig. 27. The " Devil's Cheese King," near Lynton, North Devon.
102
DYNAMICAL GEOLOGY.
[sec. ii.
cooling from without in the evening air, contract more ex¬
ternally than within, and the unyielding interior forces off
the outer parts to a distance of 1 or 2 feet. Let anyone in
a rocky place observe the fragments that have thus been
shot off, and he AviU find in the vicinity pieces from a few
ounces to ICQ or 200 lbs. weight, which exactly fit the new
surface of the original block ; and he may hear in the
evenings among the hills, where sound travels readily, the
ringing echo of the report, which the natives ascribe to the
Mebesi, or evil spirits, and the more enlightened to these
natural causes."
In warm climates, the continued heat of the summer
sun sometimes produces enormous cracks in the earth. In
the Sierra Nevada of Southern Spain, where there are dis¬
tricts composed of loose tertiary marls, great fissures have
been formed several miles in length, two or three hundred
feet deep, and several hundred yards in width. These
ravines Professor Ansted describes as having originated in
sun-cracks, though they may possibly have been deepened
by the winter rains subsequently directed into them.
Great and rapid vicissitudes of temperature, by loosen¬
ing the component particles of rocks, and causing the for¬
mation of cracks, thus facilitate the action of the more
powerful agencies of rain and wind.
Action of Frost.—The destructive action exerted by
freezing water is due to a peculiar property which it pos¬
sesses, wherein it differs from most other fluids. Most
other bodies contract in bulk as they cool down from a
fluid state to the point of solidification ; water follows this
law of contraction till it reaches the temperature of 39j°
Fahrenheit (4° Cent.) in the downward scale, it then begins
to expand, and continues to do so until it becomes solid at
the temperature of 32° Fahr, or 0° Cent.
If, therefore, rain or other water soaks into the substance
of rocks and fills up the interstices, cracks, and crevices
which they contain, and this water then freezes, its conver¬
sion into ice is accompanied by an expansion which exer¬
cises an irresistible mechanical force and produces impor¬
tant geological results. Not only are the particles of the
rock loosened and disintegrated, but the expansion of the
freezing water in the larger cracks and fissures, by which
CHAP. VI.]
TERRESTRIAL AGENCIES.
103
all rocks are traversed, causes tke fracture and displace¬
ment of large blocks, which are ultimately rent asunder
and broken up into smaller pieces.
It is on the sides and summits of mountains, which are
subject to great vicissitudes of temperature, that this
agency acts with the greatest power and effect. The hardest
rocks are broken up by it, and enormous masses are dis¬
placed and toppled down precipices, or are set rolling down
slopes to suffer still further fracture, and produce still
greater ruin in their fall. Its effects may be seen and its
amount measured by the piles of angular fragments of all
sizes, which are found at the foot of the crags and precipices,
and which sometimes form talus- and débris-slopes many
hundred feet in height.
Mr. J. F. Campbell thus describes the action of frost in
Scotland :—" The silence of the still frosty night is some¬
times broken by loud sharp reports followed by the hoarse
rumble of stones falling from the mountain peaks. Eain-
water sinks into every open chink, and when it freezes the
crystal wedge shoves great rocks from their base. The
tool-mark is an angular fracture whose shape depends on
the nature of the rock which is broken. The finished work
is to be seen in high mountains, whose tops are riven peaks,
and whose sides lie shivered where they fell." '
Captain Beechey in his " Voyage towards the North Pole "
describes the amount of this action as very great in Spitz¬
bergen. He found that the mountain sides were under¬
going rapid disintegration from the absorption of wet
during summer and its expansion by frost in winter.
" Masses of rock were in consequence repeatedly detached
from the hills, accompanied by a loud report, and falling
from a great height were shattered to fragments at the
base of the mountain, there to undergo a more active
' disintegration."
In describing the various erosive agencies now acting
upon the slopes and summits of the Rocky Mountains, Mr.
Clarence King thus describes the work done by frost
and changes of temperature on the summit peaks r—" The
whole peak region is seen to be riven with innumerable
^ " Frost and Fire," vol. ii. p. 181.
' " Exploration of the 40th Parallel," vol. i. p. 471.
104
dynamical geology.
[sec. ir.
cracks, whicli are due to unequal expansion and contrac¬
tion in a region alternately chilled hy radiation and -warmed
by the sun. Upon the steep slopes and sharp blade-like
ridges, the results of such Assuring, together with the
leverage of expanding ice, have the effect to dislodge large
fragments of rock, and produce immense slopes of debris."
He observes that this action takes place at all hours of the
day, " but especially when a sudden chill (as during the
hour after sunset) has the effect of congealing the perco¬
lating waters ; " and he adds—" Upon the summits of the
Rocky Mountains, the Uinta, and Wahsatch, and at very
many points of the ranges near the Pacific coast, I have
heard during the day thousands of blocks dislodge them¬
selves and bound down the slopes. The resulting accumu¬
lations of debris form a very conspicuous feature through¬
out the Cordilleras. In many instances they must amount
to fully 1,000 feet in thickness. In the Sierra Nevada,
where all these phenomena are on a grander scale, I
have seen debris-slopes measuring 4,000 feet from top to
bottom."
The summits of the Alps present similar appearances,
those which are not covered with perpetual snow termi¬
nating in jagged peaks and narrow points which are some¬
times called aiguilles or needles. On these exposed peaks
disintegration proceeds at a rapid rate, and showers of
stones faU continually night and day, but especially in the
afternoon when the sun's rays are hottest and the expand¬
ing force is consequently greatest.
The accompan3ring sketch, fig. 28, representing the forms
of some of these peaks, with piles of debris at their bases,
is taken, by permission, from Professor Bonney's " Alpine
Regions."
General Result of the Destruction of Rocks.—Of
all the disintegrating agencies above mentioned, rain is the
most important and universal ; there are very few districts
on the face of the earth which are absolutely rainless, and
wherever it falls it operates both chemically and mechani¬
cally over large areas at once. There are, therefore, few
rock-surfaces in the world which are not affected by this
agency, assisted in most cases either by extreme heat or
extreme cold, or by both combined.
CHAP. VI.J TERRESTRIAL AGENCIES. 105
The consequence of the erosiye action of these atmos¬
pheric agencies is the production of a superficial covering
of disintegrated rock or soil ; but this soil is being con¬
tinually removed from all sloping surfaces by the mecha¬
nical action of rain, and is thus constantly travelling to lower
and lower levels,¡while fresh rock-surfaces above are being as
constantly exposed to further disintegration by the weather.
The general result of this continual action is that all land
Fig. 28. Dolomite Peaks in decay.
surfaces are being slowly, but surely, wasted away, and the
soil, particle by particle, is being carried away into the
rivers, and by them into the sea. It is clear, therefore,
that were there no counteracting influences of upheaval,
such as have been mentioned in Chapter IV., the conti¬
nents of the primeval world would have been long ago
destroyed, and a universal but shallow ocean would now
envelope the surface of the globe.
This decay and wasting of the surface is, however, by no
106
DYNAMICAL GEOLOGY.
[sec. ii.
means uniform ; it is most rapid in countries whicli have a
large rainfall, and least appreciable in those which enjoy
a dry and equable climate, like that of Egypt. Moreover,
it is always greater in mountainous regions than on plains
and gentle slopes. Again, as we have seen, some rocks
yield much more rapidly than others, and this difference in
the rate of waste produces inequalities in the surface, which
are especially noticeable where two rocks of very different
constitution are brought into conjunction with one another
at the surface ; one will always be worn down to a lower
level than the other.
It should also be remarked that the rapidity with which
different rocks yield to the weathering agencies has no
relation to their hardness. Where other conditions are
equal, soft sandstone resists their action better than the
hardest limestone, because the latter yields so readily to the
chemical action of carbonated waters, which have no effect
upon the former. We have seen, too, that even such hard
rocks as granite and gneiss are often decomposed and dis¬
integrated in situ to an astonishing depth.
Definition of Terms.—It has been found convenient
to employ a general term to express the result accomplished
by the combined action of all these agencies, and in most
text-books the word Denudation is used in this sense, being
used to signify the removal of material from the surface of
the land. The author has long felt dissatisfied with this
use of the term denudation, because it is a perversion of its
proper and original meaning. To denude (Latin denudare),
is to uncover or lay bare a surface, and though such de¬
nudation necessarily involves the removal of matter, it is
not this matter, but the underlying surface which is de¬
nuded ; consequently it is incorrect to speak of denudation
in the sense of wearing away and removing material with¬
out reference to the uncovering of any particular stratum.
Degradation has sometimes been used in this general sense,
but it is open to the objection that it has already acquired
a different meaning in the English language. Detrition
(from detero, to wear down), has not hitherto been used as
a technical term in geology, though the words detritus and
detrital are frequently employed, and attrition is a common
English word. The word detrition, therefore, is not a great
CHAP. VI.J
TERRESTRIAL AGENCIES.
107
innovation, and in proposing it as a general term to express
the result of the processes concerned in producing detritus,
and in removing material from the surface of a country, I
hope for the support of all geologists who appreciate the
value of precise language.
We may then frame the following definitions :—
1. Detrition is the removal of rock-material from the
land.
2. Denudation Ï& the laying bare of rock-surfaces by the
detrition of the overlying beds.
3. Erosion is the localized action of any detritive agent.
4. Deposition is the disposal of the detritus removed from
denuded surfaces.
Consequently we shall be correct in speaking of
The detrition of a district.
The erosion of valleys.^
The denudation of fresh rock-surfaces.
The transportation and deposition of detritus.
' By American geologists the term corrasion is often used for the
erosive action of a running stream.
CHAPTEE VII.
UNDERGROUND CIRCULATION OF WATER.
LAEGE proportion of the rain-water which falls upon
the earth sinks beneath the surface of the soil, and
after percolating through the rocks below for a greater
or less distance, is thrown out again in the form of springs.
The depth to which it sinks depends on the nature of the
rocks which it encounters in its downward passage. Some
rocks are much more pervious than others ; sand and sand¬
stone are pervious by reason of their loose and porous
nature ; limestone and other hard rocks by reason of the
numerous cracks and joints by which they are traversed ;
while all kinds of clay and shale are comparatively imper¬
vious to water, though their constant wetness shows that
they are not absolutely impermeable.
Causes of Springs.—The position of springs is there¬
fore always determined by the nature and relative position
of the rock-beds, and is generally due to one of the fol¬
lowing causes. The underground water is brought again
to the surface—
1. By meeting with an impervious stratum.
2. By meeting with a fault or line of displacement.
3. By meeting with some open crack or fissure.
4. By the simple force of hydrostatic pressure.
5. By accumulation in absorbent strata.
I. If rain falls on a pervious rock it descends verticaEy,
but if it reaches a more impervious stratum below, it flows
laterally along the top of this bed and breaks out in springs
along the line where this comes to the surface of the
groimd. The strength of the resulting springs wül depend
CHAP. VII.J UNDERGROUND CIRCULATION OF WATER. 109
upon the relation between the slope of the ground and the
slope of the rock-beds beneath.
Fig. 29 is an imaginary cut or section through a tract of
country composed of sand or soft sandstone (a, a), under¬
laid by a stratum of clay (c, c), the surface of which slopes
(or dips) slightly towards the east. It is clear that the
rain which falls on the western hill will pass down through
the sand, and flowing outwards along the surface of the
clay will issue in springs along a Jine on the eastern side
of the hill, s\ being a point on this line.
That there should also be a spring at s^on the western side
is not at first sight so obvious, since the fall of the beds
is towards s', and water cannot run up hill : it must be re¬
membered, however, that in passing downwards and out-
Fig. 29. Origin of Springs.
The line of saturation is indicated by the dotted line.
wards through the sandstone, the water will have to over¬
come a considerable amount of friction, and that this fric¬
tion will tend to hold the water up under the hill. If,
therefore, the rainfall be tolerably constant, the water will
be accumulated in the lower portion of the sandstone more
quickly than it can escape by the springs, and consequently
a certain thickness of the rock will be in a state of constant
saturation. The thickness of the saturated portion will
decrease towards the sides where the water finds egress, and
its upper limit will form a curved line ; this is called the
line of saturation, and when this line rises above the level of
the point s^, the hydrostatic pressure will force the water
outwards, and cause, a spring at that point as well as at s'.
The former, however, will of course be a weaker spring, and
will be the first to fail in a dry season, when the line of
saturation under the hill is gradually depressed.
110
DYNAMICAL GEOLOGY.
[sec. ii.
2. The instance above given is a very simple case, though
it is one of frequent occurrence in nature. In other cases,
however, strong springs do not occur where they might be
expected on the above principle, the regularity of the under¬
ground waterflow being interfered with by the existence of
open cracks and fissures. It frequently happens that the
rocks have been more or less vertically displaced along the
plane of such cracks, so that the continuity of the several
strata is interrupted. In fig. 29, p p is a line of fault or
displacement, on the eastern side of which the rocks are
thrown up to a higher level, so that a face of clay is brought
against the lower part of the sandstone on the western side.
In this case, though there may be springs at the point s',
yet the main body of water percolating through the sand¬
stone will pass laterally along the line of fracture, and
will be thrown out at some point, s\ along its course where
h
Fig. 30. Diagram to illustrate the conditions of Artesian Wells.
the surface of the ground descends below the level of the
saturation line.
3. Artesian wells and springs are due to the confinement
of water in an inclined permeable stratum lying between
two impermeable strata, the water being prevented from
rising to the normal level by the upper impervious stratum,
until the pressure forces it up along some line of weakness,
or a boring is made down to the water-bearing stratum.
Fig. 30 shows the conditions that are necessary for the rise
of water in artesian wells ; a and c being impervious strata,
and b a pervious rock which rises into higher ground than
the clay lands on each side. The rain falling on b fills the
underground extension of that stratum up to the broken
line between a and c. If, then, c is pierced by a boring at
w, the water in b will rise to the height of the water-Une in
that stratum.
Natural artesian springs may be formed through cracks
CHAP. VII.] UNDERGROUND CIRCULATION OF WATER. Ill
in the upper stratum when the pressure is great, as in cases
where the water-bearing stratum descends below the level
of the sea. Such springs occur in the north-east of Lin¬
colnshire, where they are known as " blow-wells." These
conditions are shown on the right in fig. 30, where I is the
sea-level and s represents a crack or fissure through which
the water rises to the surface.
4. Springs sometimes issue from hill-sides without any
obvious cause for the expulsion of the water. If, however,
rain-water, after sinking for a short distance into the hill,
meets with some horizontal crack or plane of division, it
may find it easier to pass along it than to penetrate further
downwards. The hydrostatic pressure of the accumulating
waters above and behind will force the water along the
crack, even if it should slope slightly upwards toward the
surface.
5. Springs may also be caused by simple hydrostatic
pressure overcoming the force of gravity, under the follow¬
ing circumstances. In some districts the rocks are of such
a nature that water may slowly descend through them for
several thousand feet without meeting with any impervious
stratum or great line of fracture. At such great depths,
however, the rocks become denser and the cracks and
joints much narrower and fewer, so that it may become
very difficult for the water to work its way further down¬
wards ; it will then accumulate drop by drop, and the pres¬
sure of the descending supply will become very great ; the
water will work its way through lateral cracks and joints,
and if it meet vrith some large fissure or fault opening up¬
wards, it may be easier for it to ascend than to sink
further ; in other words, the hydrostatic pressure may be
sufficient to drive it up to the surface again, where it would
issue in powerful and perennial springs. Waters which rise
from great depths in this manner generally have a high
temperature, like those of Bath, and are knovm by the name
of "thermal springs"; but they must be distinguished
from the thermal springs which occur in volcanic districts,
and are hot in consequence of the high temperature of the
rocks near volcanoes.
6. Intermittent Springs.—If a pervious stratum is of
small extent, and has no subterranean outlet, the water
112
DYNAMICAL GEOLOGY.
[sec. xi.
■which gains access to it will accumulate until it reaches
a height from which it can overflow, finding its way to
the surface by the nearest outlet. Again, if the channels
of ingress are able to supply a greater quantity of water
than the ordinary channels of egress can carry off, some
other outlet at a higher level will act as a waste-pipe to
carry off the superabundant supply and an intermittent
spring will be formed.
The nailbournes and winterbournes of the south of Eng¬
land are caused by a gradual rise in the line of saturation
under the chalk-hills till the water reaches a level at which
there is free egress into some valley or depression ; this, of
course, only occurs after periods of great rainfall, and it is
only then that these " bournes " begin to flow.
Many dry valleys in chalk districts maybe regarded as the
Fig. 31. Diagram to illustrate cause of Winter-bournes.
flood-courses of subterranean streams, that is to say, the
bottom of the valley is above the level of saturation in ordi¬
nary seasons, and the water makes its way underground
through the chalk, but after long-continued rains the level of
saturation rises, and the water issues to the surface at some
point higher up the valley than the usual spring-head, often
indeed at several successively higher points. Fig. 31 re¬
presents a vertical section along such a valley, v v being the
slope of the valley bottom, and the line a c indicating the
level of saturation in ordinary seasons, so that there is
generally a spring at a. Suppose however that a wet
season raises the general water-level to the height h d, this
will cause water to issue at successive points between a
and b. The line h h is intended to represent the average
elevation of the chalk downs above the bottom of the
valley.
h
V
V
CHAP. VII.J UNDERGROUND CIRCULATION OF WATER. 113
Sucli a rise in the water-level can be measured by its
rise in deep wells ; thus a bourne sometimes issues at a
place called the Seven Barrows, near Lambourn in Berk¬
shire, and a well near by then overflows, though in dry
seasons it is sometimes dry ; this well is 73 feet deep. At
other places where bournes issue in Berkshire, Mr. P. J.
Bennett has found the rise of the water in wells to be from
70 to 100 feet.
Action of Underground Water,—The waters which
sink below the surface and flnd a passage through the
rocks exercise both a chemical and mechanical action.
The power of carbonated water to dissolve certain sub¬
stances has already been explained (p. 85), and it is evident
that this chemical energy will enable the percolating waters
to take up any soluble constituents of the rocks through
which they pass. The consequences of this solvent power
are twofold: not only will the water become charged
with many mineral substances in solution, such as the car¬
bonates of lime, iron, and magnesia ; but the pores, spaces,
and cracks within the rocks will be widened and enlarged
by the removal of these materials. The results of this
chemical action are most conspicuous among calcareous
rocks, through which long underground caves and passages
have been excavated by the flow of subterranean waters.
As a mechanical agent, underground water acts partly
as a lubricator, and partly by removing small particles of
the rock in suspension, either because they have previously
been loosened by its chemical influence, or because the
rock is of a loose and incoherent nature like sand. The
results of this action are most evident along cHfEs and
slopes, especially where conditions are favourable to the
formation of springs. Cracks are widened and particles
are removed tiU the cohesion of large masses is so weakened
that they fall or founder down to lower levels. Such dis¬
ruptions are called landshps.
Chemical KfFects : —Matter removed in Solution.—The
materials abstracted from the rocks through which water
has percolated are, of course, found in solution when the
water issues to the surface again in the form of a spring.
All well and spring waters contain a certain amount of
mineral matter, and this is held in solution until the water
I
114
DYNAMICAL GEOLOGY.
[sec. ii.
is exposed to evaporation in the air. The quantity of such
mineral matter varies greatly ; in ordinary springs of
drinkable water rising from calcareous rocks there is, on
the average, about 30 parts per 100,000, or 21 grains in a
gallon of water, but sometimes there is as much as 90
parts (or 68 grains) in a gallon. Springs which contain a
larger quantity of such matter in solution are generally
called mineral springs, and they often taste sensibly of
some of the saline ingredients ; in these the amount varies
from 100 to 3,000 parts per 100,000, i.e., from 70 to 2,100
grains per gallon.
The substances most commonly found in spring-waters
are :—
Carbonate of lime ) making temporary hard
„ magnesia J water.
,, iron making chalybeate water.
Sulphate of lime 1 making permanent hard
,, magnesia / water.
Chloride of sodium ) , . ,
> making salme water.
„ magnesium J °
Carbonate of soda 1
Sulphate of soda j °
Waters which rise from great depths, or which traverse
heated rocks in volcanic districts, acquire a high tempera¬
ture, and are then able to dissolve silica and other less
soluble matters ; moreover, the presence of alkaline car¬
bonates (i.e., those of Soda and Potash) facilitates the solu¬
tion of silica, and enables the water to hold a larger quan¬
tity of it in solution. Springs of warm water are often
called thermal springs.
The amount of material thus abstracted from the rocks
and carried away in solution is very great, as will appear
from simple calculations based on the analyses of the
waters of various springs. Two instances will suffice : (1)
BischofE says that there are fifty springs near Carlsbad,
giving out 800,000 cubic feet of water in twenty-four
hours, from which a mass of stone weighing 200,000 lbs.
could be deposited in that time. Carlsbad itself is built
upon a mass of peculiar calcareous rock deposited by these
springs, and known locally by the name of Strudelstein
CHAP. TII.J IJNDEEGEOUND CIECULATION OF WATEB. 115
the whole valley is, in fact, filled up with similar deposits.
(2) The Bath wells yield 181,440 gallons a day, and each
gallon contains from 144 to 168 grains of mineral matter
in solution ; the smaller amount is equal to 3,732 lbs. per
day, or 608 tons in a year. From these figures Professor
Ramsay has calculated that if the mineral contents could
he precipitated and solidified into a rock like limestone,
the amount annually discharged would form a column 3
feet square and about 912 feet high. The following are
the ingredients of the Bath water according to Professor
Formation of Holes, Pipes, and Caverns.—The dissolving
action of rain-water is also made apparent by the holes
and hollows which occur in the surface of all limestone dis¬
tricts ; any little depression where water can lodge is made
deeper, the joints in the rock below are widened, and a
funnel-shaped cavity or swallow hole is the ultimate result.
If the surface is covered by any kind of soil, with a
growth of trees and plants which push their roots down to
and into the rock, the rain-water is continually guided by
these roots to certain points. The rock is consequently
dissolved more rapidly below these points, and a hollow or
pipe is gradually formed, into which the overlying soil
sinks. Long chimney-like holes are sometimes produced
in this way, filled with materials derived from the surface
soil and descending for a long distance downward through
the rock. Small pipes and pockets may be seen in many
chalk-pits, varying in depth from a few feet to many yards,
and generally filled with gravel, sand, or loam derived
J. Attfield :—
Carbonate of lime
Sulphate of lime
Nitrate of lime .
Carbonate of magnesia
Chloride of magnesium
Chloride of sodium .
Sulphate of soda
Sulphate of potash .
Nitrate of potash
Carbonate of iron
Silica
Grains per gallon.
7-84
9410
■56
•56
15-24
1515
2314
6-70
105
1-21
2-70
168-25
116
DYNAMICAL GEOLOGY.
[sec. ii.
from the overlying soil. Excellent instances are visible in
the chalk quarries by the side of the Great Northern Eail-
way at Hitchin. Several huge funnel-shaped pipes, filled
with gravel, are here seen descending vertically into the
chalk, and passing down below the level of the railway.
These pits or pipes are still in process of formation.
Dr. Darwin says : ' " During the last forty years I have
seen or heard of five cases, in which a circular space, several
feet in diameter, suddenly fell in, leaving on the field an
open hole with perpendicular sides, some feet in depth.
This occurred in one of my own fields whilst it was being
rolled, and the hinder-quarters of the shaft-horse fell in ;
two or three cartloads of rubbish were required to fill up
the hole. . . . The rain-water over this whole district sinks
perpendicularly into the ground, but the chalk is more
porous in certain places than in others. Thus the drainage
from the overlying clay is directed to certain points, where
a greater amount of calcareous matter is dissolved than
elsewhere."
In districts where there is little or no soil, the swallets
or pot-holes remain open, and are often wide and deep
cavities into which the surface waters run and disappear.
Such holes are common in the limestone districts of York¬
shire, and an account of them has been published by Mr.
L. C. Miall, who mentions Thund Pot and Hellen Pot as
among the largest.^
There would appear to be two modes in which such
swallow holes can originate. Some are due to direct solu¬
tion from surface waters, and occur singly or in scattered
groups ; others occur in linear groups, and are due to un¬
equal subsidence over a line of fissure. The latter are
cWefly found where the limestone has a covering of shale,
clay, or peat ; the fissures below are then widened by
aqueous solution, till the roof can no longer support itself
at certain points, but falls in and pits the surface ; the re¬
sult is a succession of swallow holes, in which streams are
often engulphed.
Mr. "V^itaker remarks that in Berkshire there are always
swallow holes at or near the junction of the chalk and ter-
^ " Vegetable Mould and Earth Worms," p. 137.
' "Geol. Mag.," 1870, Ser. 1, vol. vii. p. 513.
CHAP. VII.J UNDERGKOUND CIRCULATION OF WATER. 117
tiary beds, and that the small streams flowing off the latter
often rim into them ; sometimes disappearing into a funnel-
shaped hollow, sometimes flowing into a small pool, the
level of which never alters, notwithstanding the continual
flow of water.'
In other countries swallow holes occur on a still larger
scale. Dr. Emil Holub describes a thick limestone forma¬
tion as extending over hundreds of square miles in the
Transvaal and Griqualand West (South Africa), and notices
the frequent occurrence over this tract of funnel-shaped
chasms, sometimes as much as 200 or 300 yards in circum¬
ference, and from 25 to 40 feet in depth. " At first sight,"
he says, " they have the appearance of being circular, but
on investigation they nearly always prove triangular or
quadrilateral. The interstices of broken rock with which
their inner surface is lined are filled up by the surrounding
earth, which thus forms a luxuriant bed for the roots of
trees and shrubs, which tower up above, and become con¬
spicuous upon the generally barren plain." From the
bottom of these funnels fissures and underground passages
radiate in different directions, and Dr. Holub describes his
descent into one which bears the name of Wonderfontein.
Prom others the water does not appear to escape so readily,
and consequently the funnels form small lakes or pools of
clear water
Suiterranean Channels and Caverns.—The underground
waters are often gathered into streams of considerable size,
and surface rivers are sometimes engulphed in rifts or
caverns, and flow for long distances in subterranean channels
before they again emerge to the surface.
Such is the stream which traverses the great caverns of
the Peak in Derbyshire.
In Staffordshire the river Manifold gradually dwindles
away in its limestone bed till the water finally disappears
in a fissure near Grindon, and after flowing underground
for a distance of three miles, reappears near Ham.
In Yorkshire, the river Aire issues from Malham Tarn,
' See also J. Prestwieh, " On some Swallow Holes near Canter¬
bury," " Quart. Journ. Geol. Soc." vol. x. p. 222.
' " Seven Years in South Africa," by Dr. Emil Holub, vol. i.
p. 170.
118
dynamical geology.
[sec. ii.
a circular lake about a mile in diameter, on the summit of
a lofty moor, aud after flowing a short distance it descends
into a subterranean channel, whence it issues again at the
foot of Malham Cove, a perpendicular limestone rock 288
feet high.^
Sometimes, when the courses of such rivers are not far
below the surface of the ground, a portion of the roof falls
in, and leaves a deep trench, at the bottom of which the
brawling waters of the stream may be distinguished among
the masses of fallen rock. Good instances of this kind
occur in the courses of the streams which flow beneath the
great limestone plains of central Ireland, and have been
thus described by Mr. Jukes : ' " Sometimes the roof of
one of the subterranean river-courses has fallen in for
some distance, so as to form a narrow rocky valley, from a
hundred yards to a mile or two in length. The river comes
from under the limestone mass at one end, and runs below
it at the other, the rock at either extremity of the fallen-in
valley rising like a wall along the face of some strong joint.
In the country, above and below one of these excavations,
no one would have any suspicion of the course, or even of
the existence of the subterranean river."
Instructive examples occur also in Greece. " It appears
that in the more elevated districts of the Morea there are
many deep land-locked valleys or basins, closed round on
all sides by mountains of flssured and cavernous limestone.
When the torrents descending from the surrounding heights
are swollen by the rains, they rush into the enclosed basins ;
but instead of giving rise to lakes, they are received into
gulfs or chasms, called by the Greeks Katavothra, which
correspond to what are termed swallow holes in the North
of England." ' In summer it is possible to penetrate far
into these chasms, and within is found a suite of chambers
communicating with each other by narrow passages. The
outlets are usually near the sea-shores of the Morea, but
are sometimes situate beneath the sea.
To the solvent action of such streams then, aided doubt-
^ Conybeare and Phillips, " Geology of England," p. 397, note.
' " Manual of Geology," third edition, p. 458.
' M. Boblaye, in Lyell's " Principles of Geology," tenth edition,
vol. ii. p. 516.
CHAP. TH.] UNDERGROUND CIRCULATION OF WATER. 119
less by tbe influence of waters percolating through the
roof, must be ascribed all the underground passages and
caverns which occur in so many parts of the world, and
which are sometimes of great size and length.
The caves of Franconia, in Germany, have for many
centuries been celebrated for their beauty, and for the
fossil remains which they contain. One of the most re¬
markable is the Cave of Gaylenreuth, the entrance to
which is in the face of a perpendicular rock on the left
bank of the Wiesent. This opening (a in fig. 32) leads into
a series of chambers, which are from 15 to 20 feet high,
O
Fig. 32. Ideal View of the Cave of Gaylenreuth.
and extend for a distance of several hundred feet ; they
terminate in a deep chasm (6), which is partially filled up
with deposits of earth enclosing the bones of the various
animals which formerly inhabited the cavern, bears, wolves,
hyaenas, etc. These cave-deposits will be described on a
future page (Chapter XI).
One of the largest caverns in the world is that at
Adelsburg, near Trieste, consisting of long series of lofty
grottoes and connecting passages through which the river
Poik now flows. It has been explored for a distance of
more than a mile and a half, the floor being then occupied
by a subterranean lake : the largest grotto or hall is about
120
DYNAMICAL GEOLOGY.
[sec. n.
180 yards from the entrance, is 300 feet long, and about
100 feet high, and is adorned with translucent stalactites
of fantastic forms. The river is believed to continue its
subterranean course for about eight miles.
The cavern of the Peak, in Derbyshire, is 2,250 feet in
length, and consists of a succession of narrow passages and
lofty chambers, some of the latter being 200 feet broad,
and more than 100 feet high. Clapham Cave, which nms
a long distance beneath Ingleborough, in West Yorkshire,
is another remarkable instance.
The Mammoth Cave, in Kentucky, U.S., is well known.
This has many branches extending in different directions,
and one of them has been followed for a distance of ten
miles without reaching its termination. Like other caverns,
it is a system of large cavities connected by narrow tunnels ;
one of the chambers occupies an area of no less than ten
acres, and has a height of 160 feet in some places.
It is not only in ancient limestones that such caverns
occur; they abound in the limestone rocks of elevated
coral islands, where the excavation of such cavities must
have been comparatively rapid. The Rev. J. Williams
states that in the island of Atiu, one of the Sandwich
Islands, there are seven or eight of large extent. He
entered one, and wandered a mile along one of its branches,
without finding an end to its " interminable wanderings." '
The island of Barbados in the West Indies affords a
curious case of a subterranean system of drainage. This
is a raised coral island, and six-sevenths of it are covered
with coral-rock, which varies in thickness, but is often
from 150 to 200 feet thick. On this area there are no
permanent surface streams, all the rain falling on it sinks
into the limestone rock, and is carried off by underground
channels, most of which open into the sea by submarine
outlets, whence fresh-water springs issue into the sea.
The coral rock seems to be traversed by a veritable net¬
work of channels and caverns ; some of these are dry, and
can be explored for long distances ; others are still occu¬
pied by the streams which made them, and they exhibit
the usual features of limestone caverns.
' See Dana's " Coral Reefs," p. 361.
CHAP. TII.J UNDEEGROUA'D CIRCULATION OF WATER. 121
Mechanical Effects :—Landslips.—Any slope of
clay or shale has a tendency to slip after heavy rain, as
may be seen in many of our railway cuttings. In dry
weather the contraction of the clay leads to the formation
of internal cracks, and when rain falls the water flows down
these, removes particles from their sides, and so lubricates
the contiguous surfaces that portions slip and founder down
by their own weight. The foundering of clay slopes can
be well studied in the Isle of Sheppey, where large portions
of ground slip seaward every winter. The manner in
which these slips are produced has been thus described : '—
" In the hot weather the surface of the ground is cracked
from the shrinking of the clay, and the cracks are seen as
gaping fissures about six inches across and many yards in
length and depth. When the rain falls, it find its way
into these cracks and softens the clay at the bottom. If
the crack is near the cliff, the half detached mass being
now heavier, owing to the addition of the water, while the
cohesion of the attached portion is lessened, slips down to
the shore below, sometimes almost unbroken, sometimes
breaking off by parallel cracks into a series of terraces, or
even in part creeping down among the fallen stiffer masses
as a glacier-like mud-fiow. The sea washes this debris
away from the base of the cliff, and the process is re¬
peated."
In harder rocks the open cracks and joints afford access
to the rain-water, and frost often assists in the process.
Professor Heim has recently called attention to the
importance of these massive landslips {Bergstürze) in
mountainous regions; and the Eev. A. Irving has dwelt
upon the part which they play in the formation of many
lakes.°
Three principal types of such landslips may be recognized
in Switzerland:—
1. Cases in which thick accumulations of debris upon
mountain sides become saturated with water after heavy
rains. The water, by increasing the weight of the mass,
and diminishing the friction, causes the whole to slide
^ Professor Hughes, in Whitaker's " Geology of the London
Basin," Mem. Geol. Survey, vol. iv. p. 387.
^ " Geol. Mag." Dec. 2, vol. x. p. 160.
122
dynamical geology.
[sec. ii.
down tlie slope. " One of the most remarkable instances
of this class is the movement which is now going on at the
village of Fetan, in Unter Engadin. Houses are tilted, or
parted asunder ; and the solid rock on which the tower of
the church stands is found to be but a great [detached]
mass of granite, which is slowly moving with the looser
materials in which it lies embedded." ^
2. Cases in which the strata forming a mountain slope
towards a valley on one side of it, so that portions become
loosened by the action of rain and frost, and slip bodily
down into the valley below. The falls from the Kossberg
near Lake Zug belong to this type. In 1806 large slices
broke away from the side of this mountain, and fell into
the valley of Goldau, burying villages, and causing great
destruction and loss of life. The great landslip at Elm
in 1881 is another well-known case where a portion of
the Tschingel Alp gave way and fell on to the village of
Elm 2,000 feet below, crushing the greater part of it. The
amount of fallen rock was estimated by Professor Heim
at ten million cubic metres, and each cubic metre of the
slate and shale which fell would weigh about two tons.
3. Cases in which huge masses of rock are detached from
the higher slopes, and form avalanches of rock and earth,
which descend into the valleys. Such falls frequently de¬
stroy whole villages, and sometimes lead to the formation
of lakes, either permanent or temporary ; and the bursting
of the latter causes destructive floods below the site of the
barrier. Thus, in 1512, a mass fell from Monte Crenone,
above Biasca, destroying 400 houses, and killing 600 people.
" The river Blegno was dammed up by it, and formed a
lake, which remained for three years, when the barrier was
broken through, and the whole valley, down to Lake Mag-
giore, devastated by floods." In 1618, the village of Plurs,
in Bergell, was totally overwhelmed, and still remains
completely buried.
" No Alpine valley," says Professor Heim, " is without
such heaps of debris and traditions having reference to
them. A still older series belongs to a time extending far
back beyond the range even of tradition." He particularly
^ Heim, quoted by Irving, op. cit. p. 161.
CHAP. VII.J UNDERGROUND CIRCULATION OF "WATER. 123
describes an enormous fall near Films, which is 2,000 feet
high, and appears to have taken place at a time anterior
to the glacial epoch.
The recent landslip in the valley of the Ehone (January,
1888) is another instance. Between Bellegarde and Evian,
the Ehone runs through a narrow rocky gorge, bounded
on one side by a lofty spur of the Jura, and on the other
by the precipice of Mont Vuache, along a ledge of which the
railway is here carried. From this mountain, an enormous
mass of earth and rock fell, destroying a short tunnel on
the railway, and stretching across to the other side of the
valley, so as completely to block the course of the Ehone.
The waters immediately began to gather behind the barrier ;
and had it been strong enough to dam them back for a
long time, the consequences would have been tremendous,
but fortunately the river soon forced a passage through
the mass of debris.
In the British Isles, landslips are most frequent where
the superposition of pervious upon impervious rocks cause
the issue of springs along the foot of a slope or cliff. In
such cases, the water is perpetually sapping the founda¬
tions of the upper beds, and lubricating the surface of the
impervious stratum below, till large masses founder, do"wn
at once, and form a series of confused mounds and
terraces.
The well-kno"wn Undercliff of the Isle of Wight is an
example of this, being in reality a succession of landslips,
caused by the slipping of masses of chalk and greensand
over the surface of the dark blue clay which underlies
them. This clay is locally termed " the blue slipper," but
is known to geologists under the name of gault. The fallen
masses form a series of irregular terraces between the real
cliff and the shore, and extend for a distance of six miles
along the coast. This undercliff and its formation is indi¬
cated in fig. 33, which is a diagrammatic section from
north to south (from Mantell's " Excursions in the Isle of
Wight").
The history of a single landslip, such as that of the great
Axmouth slip, which occurred in 1839, will be instructive.
The arrangement of the beds on this coast is similar to
that in the Isle of Wight—beds of chalk and greensand
124
DYNAMICAL GEOLOGY.
[sec. ii.
resting on a retentive stratum of clay, the surface of which
has a slight slope seaward. The following account is
abridged from Sir Charles Lyell's description : ^—" In 1839,
an excessively wet season had saturated all the rocks with
moisture, so as to increase the weight of the incumbent
mass from which the support had already been withdrawn
by the action of springs. Winter came on, and frost exer¬
cised its usual expansive effects on the water in all the
vertical cracks and fissures, and naturally tended to force
the masses outwards and seawards, while their slippery
basis of watery sand decreased the friction, and rendered
their descent over the surface of the clay a comparatively
easy process. These causes gave rise to a convulsion, which
began, on the morning of the 24th of December, with a
crashing noise. On the evening of the same day, fissures
were seen opening in the ground, and the walls of tene¬
ments rending and sinking, until a deep chasm or ravine
was formed, extending nearly three-quarters of a mile in
length, with a depth of from 100 to 150 feet, and a breadth
exceeding 240 feet. The tract intervening between this
fissure and the sea, including the ancient undercliff, was
carried bodily forward, and the pressure of this enormous
mass forced up the shore in front of it, producing a long,
upraised ridge, covered with large blocks of rock, which, at
the time of the occurrence, were invested with seaweed and
corallines, and scattered over with shells and star-fish.
" Far larger instances of ancient landslips, of which no
record is known, and which took place, perhaps, before
1 " Principles of Geology," tenth edition, vol. i. p. 536.
CHAP. VII.J UNDERGROUND CIRCULATION OF WATER. 125
historic times, or even before the country was inhabited by
man, may be observed in some parts of the south-west
coast of Ireland. On the coast west of Bearhaven, in county
Cork, and west of Brandon Head, in county Kerry, as also
in Derrymore Glen, between the mountains called Baurtre-
gaum and Cahirconrea, there are great cliffs, where the
rocks are thrown into positions which appeared puzzling
and abnormal, until it was ascertained that they formed
parts of gigantic landslips. Masses of land, with cliffs
800 feet high, were then seen to be nothing but a confused
heap of broken ruins, their cracks and dislocations being
superficial only, and not extending below the level of the
sea." '
^ Jukes' " Manual of Geology," second edition, p. 102.
CHAPTEE VIII.
FLUVIATILE AGENCIES.
1.—Action of Rivers.
OW a River is formed.—A river results from the
union of streams, just as one of these streams results
from the union of minor streams or brooks. The feeders
of a stream or river are called its tributaries ; the area
which is drained by the whole number of these tributaries
is called the area of drainage, and a river with its tribu¬
taries is often spoken of as a system of drainage. The
sources of supply are the rain which runs off the groimd
within the drainage area, and the springs which rise to the
surface within that area. Where a river has its sources in
moimtains that are permanently snow-clad, the melting
snow becomes a source of supply in summer. The ridge
or line of country from which the rainfaU runs off in diffe¬
rent directions is called a watershed, or water-parting.
Laws of Flow.—The rate at which water flows from a
range of hills to the sea varies greatly in different parts of its
journey, because the slope of the watercourse differs in diffe¬
rent places. The velocity of a stream at any one place depends
primarily on the inclination of the channel at that place, and
secondarily on the volume of water passing through it.
The inclination of the channel varies from a steep slope,
forming rapids, to a nearly level plain, through which the
water is only propelled by the force of the stream behind
it. The channel of a mountain torrent often has a slope
of over 200 feet per müe ; thus the Eeuss, between Auder-
matt and Amsteg, has a fall of about 212 feet per mile. As
examples of the channel slopes lower down the valley of a
CHAP. Tlll.j FlUVIATILE AGENCIES.
127
mountain river we may take the case of the Inn between
St. Moritz and Jenhach. Between St. Moritz and Samaden
(3 miles) the average slope is 65"3 feet per mile, between
Samaden and Landeck (80 miles) it is about 39'3 feet per
mile, from Landeck to Innsbruck (53 miles) it averages
13 "8 feet, and from Innsbruck to Jenbach (23 miles) about
4 feet per mile.^ When a river reaches great plains that
are not far above the sea-level its slope is often only a few
inches per mile.
Again, the velocity of a current varies with the voldme
of the streams. If two streams have the same slope for a
given distance, but there is more water in the one than in the
other, the larger stream will have the greater velocity. Also
if a stream is in flood its velocity at a given spot is greater
than when there is only the usual volume of water.
Further, not only does the velocity of a river vary at
different times and in different parts of its course, but it
also varies in different parts of the same cross-section of
the current, on account of the friction between the flowing
water and the river-bed. Thus the middle of the surface
of the stream flows faster than the sides, and the water at
the bottom travels slowest, the velocity of the bottom
water being often only half that of the surface water in
mid-stream.
With regard to the actual velocities of streams, it ap¬
pears that, except in falls and rapids, the fastest currents do
not exceed 20 miles an hour, and a current of 8 or 10 miles
an hour at the surface is a rapid; stream, capable of moving
large pebbles. It is stated that the average speed of an
ordinary river-current varies from 6 miles an hour when
the stream is high and swollen, to about 1^ mile an hour
when the water is low. The Ohio river at Cincinnati, where
its fall is 4 inches to the mile, has a mean surface speed
of If mile an hour when the water is low (i.e., 6 feet deep),
when the water is high (64 feet deep) the average current
is nearly 6 miles an hour, the actual rate in the middle
being 6'35 miles, and at points halfway toward either bank
5'85 miles per hour.^
' Prof. T. G. Bonney, " Geol. Mag." Dec. 3, vol. v. p. 54.
' Himnan's " Eclectic Physical Geography," 1889, p. 213.
128
dynamical geology.
[sec. ii.
Transport of Detritus.—Everyone knows that a
mountain torrent rolls pebbles along its bed, and that a
river in flood is turbid with the mud which it carries ; but
the geologist requires some more definite information on
this important subject. In studying the power of running
water to transport rock-material from higher to lower levels
there are three primary facts to be remembered :—
1. Loss of Weight.—All earths and stones lose one-third
of their weight when suspended in water, consequently the
force required for lifting them is only two-thirds of the
force necessary to lift them through air ; and still less
force is required to push or roll stones along the bed of a
stream.
2. Shape of the Fragments.—Rounded pebbles can be
rolled along by a current of water which would be quite
unable to move flat slabs of the same weight ; conversely,
flat slabs or flakes would sink more slowly than rounded
fragments of the same cubic contents. Flakes of mica
might therefore be transported onwards where grains of
quartz would sink, though the latter might be lighter than
the former.
3. Velocity of Current.—As already mentioned (p. 126),
the velocity of a stream depends upon the volume of
water and the declivity of the channel. An increase in
either of these factors quickens the current, and increased
velocity means increased power to transport material.
Mr. A. Tylor made experiments from which he arrived at
the conclusion that the velocity increases as the cube root
of the increase of volume on the same slope, and as the
cube root of the increase of slope if that is increased
and the volume of water is unaltered.^ We learn from
Mr. W. Hopkins that the transportive power of water in¬
creases as the sixth power of the velocity of the current.
Thus if the velocity is doubled its motive power is in¬
creased 64 times ; if its velocity is trebled, its motive
power is increased 729 times, and so on.®
Prom Mr. Hopkins' law above quoted we can express
the transportive power of streams in terms of the weight
' See "Geol. Mag." Dec. 2, vol. ii. p. 446.
" "Quart. Journ. Geol. Soc." vol. iv. p. 92.
CHAP. VIII.J FtUTIATILE AGENCIES.
129
of pebbles moved. Thus if a current of 2 feet per second
moves a pebble of half an ounce weight, a current of 4 feet
per second will move one of 32 ozs., or 2 lbs. ; a current of
6 feet per second will move one of nearly 23 lbs., and a
current of 8 feet per second will ir ove one of 128 lbs.
The detritus which is carried along by a stream has for
Fig. 34. Stream at Pont Aberglaslyn, North Wales,
a Mountain Torrenü
the most part been carried into it by rain and by landslips,
but a smaller quantity is derived from the banks on either
side, and from its bed by the erosive action of the stream
itself.
The blocks accumulated in a mountain torrent are usually
K
130
DYNAMICAL GEOLOGY.
[sec. ii.
fragments of rock whicli have fallen from the neighbouring
crags mto the water-course. These blocks, arresting the
force of the stream, are worn and fretted by the particles of
sand and mud with which the water is charged, till they be¬
come smooth and rounded by the constant attrition. Some
greater flood than usual then sets them in motion, they
are knocked and ground against one another till they are
broken up and converted into a number of smaller stones
or pebbles. These are rolled onward into the brook, where
they undergo a continuation of the same mechanical opera¬
tion, being perpetually ground into smaller and smaller
fragments, till they eventually become grains of sand and
particles of mud. In this shape they are delivered into
the river, and after many detentions by the way, are ultL
mately swept into the sea.
In a clear and shallow stream flowing over a sandy bed
the manner in which sand and small pebbles are rolled
along the bottom can often be watched. The bed generally
exhibits a number of transverse ridge^ or ripples, each with
a gentle slope along the up-stream side, and a steep slope
on the down-stream side. The particles of sand are rolled
by the current up the gentle slope till they come to the
crest of the ridge, when they drop down the steep slope into
the furrow. There they are covered up by succeeding
particles, the front of the ridge being continually advanced
as the particles from behind are pushed over it.
M. du Buat, in his work on Hydraulics, gives a table
which shows the velocity required to move detritus of
various sizes ; and this in a little clearer form for pebbles
of average rock, and extended by Professor Bonney, is
given below :—
Velocity of current. Size of material moved.
One of 3 inches per sec. moves fine clay or silt.
6 „ „ • „ fine sand.
8 „ „ „ particles the size of peas.
1 foot „ „ pebbles as large as beans (Lin.)
2 feet „ „ pebbles of 1 inch diameter.
2'82 feet „ „ pebbles of 2 inches diameter.
3 46 „ „ „ pebbles of 3 inches diameter.
4-00 „ „ „ pebbles of 4 inches diameter.
CHAP. VIII.J FLXJVIATILE AGENCIES.
131
Velocity of current. Size of material moved.
One of 4'47 feet per sec. moves pebbles of 5 inches diameter.
pebbles of 6 inches diameter,
pebbles of 7 inches diameter,
pebbles of 8 inches diameter,
pebbles of 9 inches diameter.
A current of 6 feet per second is about 4 miles an hour,
but this is the velocity at the bottom, which is about half
that of the surface in the middle. To move pebbles of
4 inches diameter requires a current of 21 miles an hour—
that is, a central surface velocity of miles an hour.
Matter in Solution.—Besides the detritus which is me¬
chanically transported by the force of the current, the
waters of every river contain a large amount of material in
chemical solution. The mineral substances which occur in
spring-water have already been mentioned, and these, of
course, will be present in the rivers that are fed by springs,
though in smaller proportions, because of the admixture of
rain-water, by which, however, some other ingredients are
introduced. The following is a list of the substances
generally or frequently found in river-waters :—
Carbonates of lime, magnesia, and soda.
Sulphates of lime, magnesia, potash, and soda.
Chlorides of calcium, magnesium, and sodium.
Silicates of potash and soda.
Peroxides of iron and manganese.
Silica and alumina in small quantities.
The amount of mineral matter dissolved in the water of
brooks and rivers varies from 15 to 50 parts in the 100,000
parts of water. The water flowing off siliceous rocks
seldom has more than 20 parts, while that running off cal¬
careous rocks generally contains over 80 parts per 100,000.
The amount in ordinary river-water may be taken as about
20 parts per 100,000, but it varies with the rainfall, being
at a maximum when the rainfall is least and the supply of
water is in consequence mainly derived from springs, and
it is at a minimum after times of heavy rainfall.
The Thames carries past Kingston 27 parts per 100,000,
or 19 grains of solid matter per gallon of water, and the
4-90 „
>>
J»
5-29 „
5-65 „
99
ft
6-00 „
99
132
dynamical geology.
[sec. ii.
mean annual discharge of water at the same place is at the
rate of 1,250 millions of gallons a day. Prom these data
Professor Prestwich has calculated that the average amount
of dissolved mineral matter carried by the Thames past
Kingston every 24 hours is 1,502 tons, which is equal to
548,230 tons in the year. Of this quantity about two-
thirds (say 1,000 tons daily) is carbonate of lime, and 238
tons is sulphate of lime, while smaller proportions of mag¬
nesium carbonate, sodium and potassium chlorides, sodium
and potassium sulphates, silica, and a little iron and
alumina, constitute the remainder.
Mr. Mellard Eeade has calculated, from the data given
in the Sixth Report of the Rivers Pollution Committee,
that all the rivers of England and Wales taken together
remove in solution 143 "5 tons of mineral matter from
every square mile per annum, or a total amount of
8,370,630 tons per annum. This at 15 cubic feet to a ton
gives 125,559,450 feet, and this is equivalent to a general
lowering of the surface by *0077 of a foot, or nearly one-
tenth of an inch in a century. Taking the world generally,
Mr. Reade computes that the quantity of mineral matter
carried off the surface of the land by rivers is about 100
tons per square mile per annum, and that of this about
50 tons may be carbonate of lime.^
In European rivers carbonate of lime generally forms
more than half the total amount of matter in solution ; its
mean quantity, according to Bischof, being 11 "34 in the
100,000. The waters of the Rhone appear to remove a
larger amount of carbonate of lime from each square müe
of drainage basin than any other European river. The total
solids in solution amount, according to Mellard Reade, to
232 tons per square mile per annum, and, according to
Bischofs analysis, about two-thirds of this must be car¬
bonate of lime.
After the carbonate the sulphate of lime is the next most
abundant mineral in solution, and in some rivers forms
nearly half the dissolved matter. The quantity in the
' "Address on Geological Time," Liverpool Geol. Soc., 1876.
The proportion of the surface area of calcareous rocks in England
and Wales is rather larger than that of Europe generally.
CHAP. VIII.J FLUVIATILE AGENCIES.
133
Thames water at Kingston is about one-sixth the total
solids in solution, according to Professor Prestwich.
Less in amount are chloride of sodium, carbonate of
magnesia, and silica. The proportion of silica found in
river-water is believed to depend upon the concomitant
amount of organic acids derived from the humus or vege¬
table soil occurring in the basin of drainage.^ In the
water of the Thames at London Bridge, the amount of
silica is stated as *1239 grains in a gallon,'' or -214 parts in
100,000 ; in the Rhine at Strasburg as much as 4'88 parts
have been found.
Total Quantities of Transported Matter.—The
calculations above given refer only to matter in solution ;
the total amount of matter carried down annually by any
river can be estimated by examining its waters at different
periods and determining their solid contents. The amount
of matter carried down by any river in flood is very much
greater than when the water is low, but Mr. Mellard Eeade
has pointed out that the amount in solution does not vary
so much as that in suspension.® The Nile when in flood
briugs down 41 times as much matter in solution as it does
during low Nile, but the amount in suspension varies,
according to Dr. Letheby, from 4 772 parts per 100,000 at
low Nile to 149T57 parts at high Nile, or 31 times as much,
the flow of water being also increased about 26 times, so
that the total amount brought down at high Nile is about
800 times as much as at low Nile.
In the case of the Danube the smallest delivery of sus¬
pended matter per day of twenty-four hours, is stated by
Sir C. A. Hartley to be 11,000 tons, the greatest 2,500,000
tons,* or a variation of 227 times.
In order to make an exact calculation of this matter four
things should be known regarding the river:—
1. The mean annual discharge of water.
2. The average amount of mud in suspension.
3. The average amount of coarser débris pushed along
the river bottom.
' SeeSterryHunt,"CheniicalandGeologicalEssays,"pp. 126,150.
^ Ashley, in " Quart. Journ. Chem. Soc." vol. ii. p. 74.
® " On Rivers," "Trans. Geol. Assoc. Liverpool," 1882.
* Reade, op. cit.
134
DYNAMICAL GEOLOGY.
[sec. ii
4. The proportion of mineral matter in solution.
These data have not yet been all carefully obtained for
many rivers, but some of them have been estimated in the
case of several rivers, with the following results.
The Mississippi carries down every year about
6,724,000,000 cubic feet of sediment in mechanical suspen¬
sion, and about 750,000,000 cubic feet more of sand and
gravel is rolled along the bottom of its channel, making a
total of about 7,474,000,000 cubic feet per annum. This
would cover a square mile of ground to a depth of 368 feet.
The amount in suspension only, calculated at 19 cubic feet
to the ton, is equivalent to 354,000,000 tons per annum, and
that rolled along is equivalent to about 39,500,000 tons
per annum. The amount carried in solution has been cal¬
culated by Mr. Mellard Eeade from an analysis of water
taken a few miles above New Orleans, which gave 15"487
grains per American gallon, in which there are 56,000
grains of water. On this basis he estimates that the river
carries 150 millions of tons of soluble matter per annum
past New Orleans.
According to the observations of Sir C. A. Hartley on
the Danube, the average amount of matter carried in sus¬
pension by this river may be put at 67,760,000 tons per
annum ; but the quantity rolled along the bottom has not
been estimated. Neither has the amount in solution been
accurately ascertained. Bischof gives an analysis of the
water near Vienna, which yielded 12*42 parts per 100,000.
Assuming this to be an average quantity, Mr. Eeade calcu¬
lates the total solids carried in solution past Vienna to be
22,521,434 tons per annum.^
The Ehone has been estimated to discharge 600,381,800
cubic feet of sediment per annum, which is equivalent to
about 30,000,000 tons, but this is probably rather too
great. Experiments at Aries gave the mean proportion of
suspended matter in the river by weight as and as
the annual discharge of water at Avignon is estimated at
about 53,144,000,000 tons, the quantity of suspended
matter calculated from this should be 26,572,000 tons per
1 Reade, " Denudation of the Two Americas," Geol. Soc. Liver¬
pool, 1875.
CHAP. VIII.]
FLUVIATILE AGENCIES.
135
annum. With regard to the matter in solution, the mean
of four analyses given by Bischoff is 15'6, which as calcu¬
lated by Mr. Eeade gives 8,290,464 tons per annum.
The Nile is stated by M. Talabot to discharge at the rate
of about 88,500 millions tons of water a year. Mr. Fowler
estimates it at 100,000 millions, and states the amount of
sediment carried to be 67 millions of tons a year.^ Dr.
Letheby found the amount in solution to vary from 13 614
to 20*471 per 100,000 in 1874 ; if we assume the mean to
be 17 parts, and take Mr. Fowler's estimate of discharge,
we find the amount of matter carried by the Nile in solution
to be 16,950,000 tons a year.
The amount of suspended sediment carried by the
Thames has never been estimated, except on the assump¬
tion that the proportion by weight to that of the water is
the same as that of the Mississippi, in which case the
amount would be 982,000 tons a year. The amount in
solution has been given by Prestwich as 548,230 tons.
Tabulating the above statements and calculations we
have—
Mississippi.
In suspension =
Along bottom —
In solution =
354,000,000
39,500,000
150,000,000
tons,
tons,
tons.
Total
543,500,000
tons.
Danube.
In suspension =
In solution =
67,760,000
22,521,434
tons,
tons.
Total
90,281,434
tons.
Nile.
In suspension =
In solution =
67,000,000
? 16,950,000
tons,
tons.
Total
83,950,000
tons.
Rhone.
In suspension =
In solution =
26,572,000
8,290,464
tons,
tons.
Total
34,862,464
tons.
• " Brit. Assoc. Kep." 1882, Address to Mech. Sec.
136
DYNAMICAL GEOLOGY.
[sec. ii.
Thames. In suspension = ? 982,000 tons.
In solution = 548,230 tons.
Total 1,530,230 tons.
Erosive Power.—That running streams have the
power of deepening and widening the channels in which
they flow, is now a well established and most important
fach The power of running water to cut out a channel
for Hself is often well exemplified on a newly-made bank
of earth or sand, before it becomes covered with vegeta¬
tion. If such a bank of earth is observed after a shower
of rain, it will be noticed that the slope is furrowed from
top to bottom by little runnels and channels, which have
been excavated by the rills into which the rain-drops col¬
lected as they coursed down the slope. This is a miniature
representation of what takes place on the sides of a valley
or the slopes of a watershed.
Sir Charles Lyell describes a ravine near Milledgeville
in G-eorgia, U.S., entirely formed by the torrential action of
rain-streams consequent upon the cutting down of the
forests, which had previously prevented the rain from col¬
lecting into large streams. When the land was cleared
cracks 3 feet deep were caused by the heat of the sun ; and
during the rains the rush of water deepened the principal
crack at its lower extremity, whence the excavating power
worked backwards, till, in the course of twenty years, a
chasm was produced no less than 55 feet deep, 300 yards
long, and varying in width from 20 to 180 feet.^
Miss Bird, describing a journey during heavy rain in the
Sandwich Islands, says : " In some places several feet of
soil had been washed away, and we passed through water-
rents, the sides of which were as high as our horses' heads,
where the ground had been level a few days before."
The process may also be studied with great advantage on
the slopes of volcanic cones which have ceased to be active.
An excellent set of examples has been described by Professor
Jukes in the cluster of cones which have been built up
inside the great crater of the Bromo in Java. Mr. Jukes
^ " Principles of Geology," tenth edition, vol. i. p. 344.
CHAP. VIII.J
FLrVIATILE AGENCIES.
137
says : * " One thing which struck me particularly in this
central group of cones (in the crater of the Bromo) was,
that the one in action was perfectly smooth and regular
outside, like a loaf of sugar, while those that had shortly
ceased to eject materials showed the irresistible progress of
denudation in the commencement of rain-gullies radiating
down their slopes. Those that were still older, as proved
by young trees having commenced to grow on their flanks,
had these gullies much deeper and broader, so that they
began to show narrow ridges, separated by small ravines
radiating from the summit, like the ribs of a half-opened
umbrella, as described by Junghun, when speaking of the
outer slopes of the greater mountains."
In the same way, the outer slopes of the volcanoes in the
South Pacific, described by Dana, are worn down to the con¬
dition of narrow ridges, with very steep walls, separated by
flat-bottomed valleys, which gradually contract towards
the interior ; and at the head of each valley may be seen a
little rill of water leaping from crag to crag, still carry¬
ing on the work of erosion for which, except in times of
flood, it seems so utterly inadequate.
It is not the mere force and impact of the descending
water which gives a stream this excavating power ; it is
the quantity of detritus, large and small, which it carries
along and hurls against the sides and bottom of its channel.
The fine sand and mud which the rain washes off the sur¬
face of the land, are the initial instruments of erosion ; but
in rapid streams and mountain torrents, the stones and
pebbles which are rolled along the bottom, as well as the
sand and mud which are held in mechanical suspension,
all help in the process of erosion. This process is there¬
fore one of mechanical wear ; the tools are pebbles, sand,
and mud, the effective force which moves them is that of
running water.
Vertical Krosion.—The erosive action of mountain
torrents can hardly fail to be perceived by anyone who
visits them and observes the beds of their narrow channels.
These exhibit smooth grooves and cuts even in the hardest
rocks, which are obviously due to attrition ; while vertical
^ "Manual of Geology,'' third edition, p. 353.
138
DYNAMICAL GEOLOGY.
[sec. ii.
pits, called pot-holes, several feet in depth, are often formed
in such rock bj the whirling action of water keeping a few
pebbles or a little coarse sand in perpetual circular motion.
Cascades and waterfalls dig deep holes and black pools
below the ledges over which they fall ; these ledges, indeed,
are often undermined by the dashing water, and are broken
off, block by block, so that the cataract recedes farther and
farther back ; while a deep ravine is left in front, extending
from the present falls to the point where they originally
commenced. Niagara is an excellent instance of such
erosion, the gorge below the cataract being seven miles in
length from the original site of the falls above Queens-
town to their present position ; and the present rate of
their recession has been recently estimate at about three
feet per annum.
Pot-holes and waterfalls illustrate the excavating power
of running or falling water when its action is concentrated
on one spot, but similar wear and tear distributed over a
certain length of the ehannel will deepen it in the same
way.
Lateral Erosion.—Mr. J. Fergusson has defined a
river as a body of water in unstable equihbrium, whose
normal condition is that of motion down an inclined plane ;
and he says, if all inequalities of surface and of soil could
be removed, it would flow continuously in a straight line,
" but any inequality whatever necessarily induces an oscil¬
lation, and the action being continuous, the effects are
cumulative, and the oscillation goes on increasing till it
reaches the mean between the force of gravity tending to
draw it in a straight line, and the force due to the obstrue-
tion tending to give it a direction at right angles to the
former." ' The result is a series of curves which, in a
homogeneous soil, would bear definite relations to the
volume of water and the gradient of the channel.
It may be stated in general terms that where the slope
is steep the stream will tend to run in a nearly straight
line ; and where the slope is shght, a stream will always
1 " Quart. Journ. Geol. Soc." vol. xix. p. 322. He proceeds to
lay down certain laws of curvature which, however, are not prac¬
tically applicable, though no doubt where other thing's are equal a
large river makes a larger curve than a smaller river would.
CHAP. Tlll.j FLUVIATILE AGENCIES.
139
take a very winding course. It is also true that there is a
point in the descent of every stream where it ceases to cut
vertically, and is occupied in cutting laterally. No rule
can be given about the position of this point, because it is
determined by the velocity of the current, and this varies
with the state of the river ; and some streams may begin
to cut latei;p,lly before they cease to cut vertically ; but for
a given stream at a given time, the place of change
might be approximately determined by observation and
the gradient noted.
Wherever a river cuts laterally, and not vertically, it is
continually changing its channel, and always leaves a flat
of deposited detritus over the portion of its channel which
it has deserted. The result of this is to cut out a broad
space, and to form a wide flat-bottomed valley, the width
of which is generally proportional to the volume of the
river. When the river is in flood, it covers the whole of
the flat, which is often called flood plain, but, when the
water is low, the stream winds about within this flood plain.
After winding in a flood plain for a certain distance, a
river may reach a part of its course where the gradient is
steeper, and will then resume the power of deepening its
channel. Hence many rivers run through an alternating
succession of deep-cut gorges and wide open valleys ; but
the subject of valley-making will be discussed more fully
in Part III. of this volume.
It must not be supposed, however, that the depth and
width of a valley depend only on the volume and velocity
of the stream ; as long as the rocks through which it makes
its way are of the same kind and degree of hardness, its
erosive effect will be chiefly regulated by these conditions.
But where, as usually happens, the valley traverses a series
of different strata, its width and depth will vary according
to the hardness and ability of each stratum to resist the
erosive action of the stream, and the detritive power of
rain.
In a humid climate like that of England and of northern
Europe generally, and in districts where the rocks are com¬
paratively soft, the valleys are broad and open, with gently
sloping sides, and the hills have rounded outlines forming
binomial curves.
140
dynamical geology.
[sec. ii.
These features are due to the disintegrating and detritive
effects of rain on the valley-sides ; the action of rain and
frost, already described, causes the banks to crumble and
moulder down, and every heavy shower of rain washes
some of the soil so produced into the stream below, by
which it is carried away to be deposited elsewhere.
In such districts, therefore, rain has as much to do with
the formation of valleys as the rivers have,'* aná the valleys
which are formed by the combined influence of these two
agencies are always widely V-shaped, the sides sloping ata
less angle in proportion as the rocks are soft and the rain¬
fall great.
On the contrary, where the rocks are hard, the erosive
action of the stream tends to excavate narrow steep-sided
valleys or U-shaped gorges, the sides of which approach
the vertical in proportion as the rock is hard and the local
rainfall is small.
In places where a softer kind of rock rests upon a harder
one, the valley will present a combination of the two forms
as represented in fig. 35.
When the banks are soft and easily eroded, and the velo¬
city of the stream is small, the conditions are most favour¬
able for lateral erosion, and a wide, flat-bottomed valley,
filled with alluvial detritus, is the result. This is well
exemplified by the lower part of the valley of the Missis¬
sippi, where the stream has lost all power of vertical
erosion, and pursues its meandering course through an
alluvial plain of its own formation, which varies from 60
to 80 miles in width.
b
Fig. 35. Erosion of Valleys.
a, hard rock.
h, soft shale.
' See "Eain and Elvers," hy Col. G. Greenwood, 1875.
CHAP. Till.]
FLUVIATILE AGEXCIE.S.
141
Rapidity of Erosion.—Prom what has already been
stated, it will be seen that the erosive power of any stream
—i.e. the quantity of material which it can remove from
its valley—depends upon four circumstances—
1. The volume of water, depending upon the annual
rainfall, frequency of floods, and the porosity of the
soil;
2. The velocity of stream, depending on the volume of
water, and the slope of the watercourse ;
3. The character of the rocks through which it flows ;
4. The load or quantity of detritus which it carries.
In considering the work done by rivers we must always
remember that the running water is only the motive power,
and that the instrument which really does the work is the
sand and mud carried along by the water. A perfectly
clear stream running over a bottom of pebbles which it
does not move is doing no work at all ; it is only when
loaded with detritus that it becomes such a powerful agent
of erosion.
The facts previously stated show how much the capacity
of a stream for lifting and carrying stones is increased by
a slight increase of velocity, and their power of erosion
must, of course, be correspondingly increased. Mr. A.
Tylor has stated that the erosive force of a stream increases
as the fourth power of the increase of velocity.^ If this is
correct,^ and his law of increase of velocity, previously
cited, is also valid, the result may be stated as follows :—
Supposing a sudden rainfall to take place in the catch¬
ment area of a river, increasing the volume of the stream
to eight times its usual quantity, the velocity would be
doubled and the river would have 16 tiînes its ordinary
erosive force ; if the volume was increased 27 times, the
river would flow with 3 times the velocity, and with 81
times its former erosive force. This explains the astonish¬
ing effects of rivers in times of flood. Large boulders are
then moved, stones and pebbles are driven against the sides
^ " Geol. Mag." Dec. 2, vol. ii. p. 459.
^ It is confirmed by Le Conte, "Geol. Mag." Dec. 2, vol. ix.
p. 288, who points out that in erosion the resistance to he over¬
come is cohesion, not weight, and thinks the power of removal wUl
vary between v' and u®, so that it is probably often as v*.
142
dynamical geology.
[sec. ii.
and bottom of the channel, and the whole work of attrition
and excavation is carried on with greatly intensified force.
Sir Charles Lyell remarks : "It is evident, therefore,
that when we are speculating on the excavating force which
a river may have exerted in any particular valley, the most
important question is, not the volume of the existing stream,
nor even the nature of the rocks, but the probability of a
succession of floods at some period since the time when the
valley may have been first elevated above the sea." '
In temperate regions these are exceptional occurrences ;
but, in tropical countries, floods are of constant occurrence
during the rainy season ; conseqnently the erosive power
of the streams is enormously increased, and all the results
of detrition are on a much larger scale. As this is a fact
which European geologists do not always realize, we adduce
the testimony of one whose experience well qualified him
to express an opinion on the subject ; ^—" In the British
Islands, the average rainfall is about 24 inches, distributed
over the greater portion of the year. In India it averages
about 50 inches, by far the greater portion of which falls in
three months. The showers are far heavier, and far more
effective in sweeping soil, sand, and pebbles from the sur¬
face of the country into the streams ; and floods in the
latter are of annual occurrence, instead of only happening
at rare intervals. The effect of a river in full flood, in
sweeping detritus down into the sea, compared with the
usual denudating action, is as the comparison of the effect
produced by the breakers of the ocean in a storm to those
of an inland sea on an ordinarily fair day.
" In flood, a river is liquid mud, rather than water. But,
in addition to Äie great floods, minor floods in tropical
streams are matters frêquently of daily occurrence during
the rainy season. After seeing, both in India and Abyssinia,
what the effects of these agents are in tropical countries, I
do not feel surprised that their powers should be recognized
with difficulty in regions where their effects are compara¬
tively so dwarfed as in the British Isles."
Heavy and destructive floods however do sometimes
' " Principles of Geology," tenth edition, vol. i. p. 354.
" Blanford, " Geology and Zoology of Abyssinia, p. 157, note.
CHAP. VIII.J
FLUVIATILE AGENCIES.
143
occur in Europe and the British Isles, and others have
been caused by the bursting of reservoirs, and though this
is not a natural cause, the results are of the same kind.
The following instances will enable the reader to realize
these results more fully.
1. Professor Green mentions some results of a flood
which was caused by the bursting of a reservoir near Shef¬
field, in 1864. He says : " At sharp bends in the valley,
where the water had impinged on projecting spurs of the
bank, or where it had been driven into a recess, it had ex¬
cavated in the solid sandstone rock large hollows, which
anyone, who was not aware of the circumstances of the
case, would have supposed to be quarries. Many acres of
meadow-land were deeply buried beneath heaps of débris,
consisting mainly of large angular blocks of rock, which
the torrent had torn off from the banks as it rushed
along." ^
2. Sir Charles Lyell has described the destruction worked
at Tivoli by the waters of the Tiber, after heavy rains, in
1826. The stream burst its usual bounds, and, impinging
on its right bank, had, in the course of a few hours, under¬
mined a high chff, and widened its channel by about
40 feet. The church of St. Lucia, and about thirty-six
houses of the town of Tivoli, were carried away, presenting,
as they sank into the flood, a terrific scene of destruction
to the spectators on the opposite bank." ^
3. Lyell also mentions theAlpbach, a torrent which falls
over a cascade near Meyringen in Switzerland, and is liable
to violent floods, by one of which the town was nearly
washed away. " This mischief happens after long droughts,
when the sun has had great power on the %late rocks above,
and caused them to crack and decompose. Those who have
not witnessed it cannot conceive the strength of water
charged with this black powder. The rocks seemed actually
to float in it, and a trifling depth of it was pushing heavy
bodies along." '
4. Floods in Scotland, 1829.—Many illustrations of the
'■ See Prestwich, in " Quart. Journ. Geol. Soc." viii. p. 224.
' " Principles of Geology," tenth edition, vol. i. p. 355.
® " Life of Sir Charles Lyell," vol. i. p. 85.
144
dynaaiical geology.
[sec. ii.
power of rivers to move stones, and deepen or widen their
valleys, were afforded by the floods which followed the
storm of the 3rd and 4th of August, 1829. The effects
were particularly apparent in Aberdeenshire. Bridges and
roads were swept away, and great accumulations of stones
and blocks of rock were left on the surface of fields, many
feet above the ordinary level of the rivers. " Some new
ravines were formed on the sides of mountains where no
streams had previously flowed ; and ancient river-channels,
which had never been filled from time immemorial, gave
passage to a copious flood."
Instances of River Erosion.—Having in the pre¬
ceding pages indicated the manner in which streams exca¬
vate and widen their channels, and the causes which assist
rivers in forming the valleys they occupy, we shall now
describe several cases in which it is clear that ravines and
valleys have been formed by rivers. The ravine at Mil-
ledgeville, described by Lyell, and mentioned on p. 136, is
an instance of the excavating power of a stream formed by
the concentration of rills of rain-water. The observations
of Professor Jukes, in Java, furnish us with an equally
convincing example.
The following are cases of river-valleys where the exca¬
vating power of running water is very clearly shown.
1. Gorge of the Simeto.—Near the town of Aderno, where
the river Simeto passes the western base of Etna, a current
of lava has descended into the valley, and, in so doing,
must have completely blocked up the course of the river.
This took place, according to Gemmellaro, in the year
1603. Through this lava, which solidified into a hard,
blue compact rock, resembling ordinary basalt, the Simeto
has excavated a ravine from 40 to 50 feet deep, and from
50 to several hundred feet wide. Sir Charles Lyell thus
describes the scene in 1828: "On entering the narrow
ravine, we are entirely shut out from all view of the sur¬
rounding country ; and a geologist, who is accustomed to
associate the characteristic features of the landscape with
the relative age of certain rocks, can scarcely dissuade him¬
self from the belief that he is contemplating a scene in
some rocky gorge of very ancient date. But the moment
we reascend the cliff, the spell is broken . . . for we stand
CHAP. VIII.J FLUVIATILE AGENCIES.
145
on the black and rugged surface of a vast current of lava,
which seems unbroken, and which we can trace nearly up
to the distant summit of that majestic cone which Pindar
called the ' pillar of heaven.' " '
This ravine, therefore, has been eroded by the stream in
the course of about two centuries ; but, as Lyell observes,
it has not yet cut down to the ancient bed of which it was
dispossessed, and the probable position of which he indi¬
cates in a diagram.
2. Valleys of Auvergne.—What has happened once in the
case of the Simeto has happened several times in many of
the valleys of Central Prance. These valleys have been
excavated to a great depth through strata of fresh-water
origin, and have been flooded from time to time by streams
a
Fig. 36. Diagram Section across Valley of Chanonat, Auvergne.
a, the Gergovia platform, b, the La Serre platform, d, the
Chanonat gorge.
of liquid lava from the neighbouring heights. Each suc¬
cessive lava-flow must necessarily have occupied the lowest
levels to which it had access ; but they now form terraces
or platforms at various heights, from 1,500 feet downwards
to the level of the present watercourses. These valleys
were originally described by Mr. Poulett Scrope, who says : ^
" Take, for instance, the two basaltic platforms of Glergovia
and La Serre, and the bed of basalt which occupies the
bottom of the intervening valley of Chanonat, Here are
three long sheets of lava, each of which, by its gradual
inclination in the same direction, and by the remains of
the volcanic vent from which it was erupted, is proved to
' " Principles of Geology," tenth edition, vol. i. p. 358.
" The Volcanoes of Auvergne," 1825, and " Geol. Mag.'" vol. iii
p. 195.
L
146
dynamical geology.
[sec. ii.
have flowed down the valley." The platform of La Serre
is everywhere from 300 to 500 feet below that of Grergovia,
and the valley must have been deepened by that depth
during the interval between the two outflows. Again, by
the same reasoning, the valley of Chanonat must have been
entirely excavated between the epoch of the La Serre lava
and the production of the more recent bed of basalt, 500
feet below. Finally, the Chanonat rivulet has worn a new
channel, from 20 to 50 feet below the level of the pebble
bed on which the most recent of the three lava currents
reposes (see flg. 36).
From this and numerous other instances, he justly con¬
cludes that " the erosive force of the streams which now
border or intersect these lava currents, together with the
action of rain, frost, and other atmospheric forces, have
alone hollowed out the extensive systems of valleys " which
are found in the province of Auvergne. Floods doubtless
occurred from time to time, and materially assisted in the
erosion ; but it is very clear that these valleys do not owe
their origin to any one unusual or cataclysmal flood, or to
any sudden disruption of the rocks.
Sir A. G-eikie has so excellently described the impression
which the valleys of Auvergne make upon the mind of a
geologist, that I venture to quote several passages in
full
" No one, whose observations have been confined to a
country which has been above the sea only since the glacial
period, or the contours of which have been smoothed over
by the ice-sheets of that time, can readily form an adequate
idea of the denuding effect of water flowing over the sur¬
face of the land. . . . The first impression received from a
landscape, like that round Le Puy, is rather one of utter
bewilderment. The upsetting of all one's previous esti¬
mates of the power of rain and rivers is sudden and com¬
plete. It is not without an effort, and after having analyzed
the scene, feature by feature, that the geologist can take it
all in. But when he has done so, his views of the effects
of subaerial disintegration become pennanently altered,
and he quits the district with a rooted conviction that there
is almost no amount of waste and erosion of the solid
framework of the land, which may not be brought about
CHAP. VIII.] FLUVIATILE AGENCIES.
147
in time by the combined influence of springs, frost, rain,
and rivers." '
3. Valleys of the Eifel.—The country of the Lower Eifel,
where it is traversed by the rivers Rhine and Moselle,
affords another excellent instance of the excavation of
valleys by river erosion. The country is composed of beds
of hard rock, bent and inclined in various directions, and
the general surface forms a gently undulating table-land,
about 1,000 or 1,200 feet above the level of the sea.
Through this country the rivers flow at the bottom of deep
and winding valleys, forming the magnificent curving
slopes which are the chief feature in the scenery of the
district. This is particularly striking in the case of the
Moselle, which works its way in so tortuous a course that
some of its bends form loops, and curve back, so as to
leave but a narrow ridge between the two reaches of the
river. This singularly winding valley is both narrow and
deep, being closely environed by steep, precipitous banks,
800 to 1,000 feet in height ; and between their opposing
bases, there is frequently no more room than is just sufiä-
cient for the river itself.
The country on either side is furrowed by numerous
naarrow ravines, with steep and often precipitous sides,
which cut down, as they near the Moselle, to depths of 600
or 800 feet below the surface of the plain, and curve from
side to side in equally sinuous courses.
The only explanation which can be given of such a
wonderful system of valleys is that the streams originally
ran over the surface of the plain, and have constantly main¬
tained the winding courses of their early channels, gradually
engraving them deeper and deeper into the land, as the
work of erosion and excavation proceeded. The action of
rain has been co-extensive with that of the streams, wear¬
ing down the valley sides, and widening them into the shape
they now possess.
4. The most wonderful instances, however, of the erosive
power of rivers are to be found in the deep and trench-like
vaUeys of the Western States of North America.
The Colorado is formed by the union of the Green and
' "Geological Sketches," by A. Geikie, 1882, pp. 93, 121.
148
DYNAMICAL GEOLOGY.
[^EC. II.
G-rand Rivers, but the Green River may be regarded as the
continuation of the main stream. This rises in Wyoming,
and is joined by the Grand in Utah, whence the Colorado
runs through Utah and Arizona to the Gulf of California.
The Uinta Mountains are traversed by the Green River in
a double series of gorges (or cañons), the deepest parts of
which have walls from 2,000 to 2,500 feet high. Then, after
running through a more open valley, the river traverses
three great inclined plateaux, in a series of cañons which
have received the names of Desolation, Gray, and Labyrinth
Cañons. It then enters Stillwater Canon, and in this,
more than 1,200 feet below the general surface of the
country, the Green and Grand Rivers unite their waters.
The Colorado, below this junction, at once enters Cata¬
ract Cañón, and proceeds through a still more wonderful
and gigantic series of gorges, which have received the names
of Glen Cañón, Marble Cañón, and Grand Canon. Glen
Cañón is 149 miles long, but is not very deep ; Marble
Canon is only 200 feet deep at its head, but steadily in¬
creases in depth, till near the lower end its walls are 3,500
feet high, and present a magnificent set of precipices, which
in some parts are nearly vertical.
The Grand Cañón is about 220 miles long, and is really
a valley within a valley, consisting of an upper trough or
depression eight to ten miles wide, with a level floor and
precipitous sides about 1,000 feet high, and an inner gorge
or cañón which varies in depth, but is in several places
from 4,000 to 5,000 feet deep, its walls being a succession
of vertical precipices broken only by short steep talus-
slopes. One of the deepest and finest parts of this canon
is represented in fig. 87. The country on both sides of
this enormous ravine is channelled by innumerable narrow,
deep, and winding cañons, all similar in their general
characters, and each one occupied by a tributary of the
main river.
These extraordinary ravines have been cut down through
horizontal or gently-inclined beds of sandstone, shale, and
limestone, and into the granite which underlies them, the
limestones being especially hard and massive, with an
aggregate thickness of from 2,200 to 2,500 feet. The
shales and sandstones lie above these limestones, and it is
Fig. 37. Part of the Grand Cañen of the Colorado.
150
dynamical geology.
[sec. ii.
these beds which have been cut back so as to leave the
broad shelf or plain on either side between the edges of the
central ravine and the lines of cliff which bound the upper
part of the valley. The outline of the whole valley is then
similar to that in fig. 35, only that the edges of the upper
portion are not sloping, but nearly vertical.^
The reason why these great ravines are such excellent
instances of river erosion is because they were made almost
entirely by the rivers alone, with little assistance from
any other detritive agencies. The district drained by the
Colorado and its tributaries is a series of elevated plateaux :
and there is little doubt that, when the course of the river
was first determined, the country stood at a much lower
level, and the tributary streams ran in channels of no great
depth. The canon district is now from 4,000 to 8,000 feet
above the sea, and the mountains which form the watershed
rise to 10,000 and 14,000 feet. These snow-clad summits
furnish a perpetual supply of water, which on reaching the
plateaus above the cañons becomes charged with sand and
mud.
Two circumstances have combined to maintain the verti-
cality of the cañón walls during the process of erosion : the
first is a decrease in the rainfall of the cañón district, for
at the present time it is practically rainless, and we have
seen that gorges are steep and narrow in proportion as the
rocks are hard and the rainfall small ; secondly, the beds
through which they are cut are for the most part horizontal,
or very gently inclined, and are traversed by strong vertical
planes of jointing, so that they break away naturally in
vertical lines.
The conditions which have enabled rivers to cut their
way through what are now ridges and mountains, the
forces which retarder increase their power of erosion, and
the limits which are set to their action, are matters that
will be discussed in the third portion of this volume.
' For descriptions and illustrations of these oafions, see " Ex¬
ploration of the Colorado River."' by Ives and Newberry, IS'il, and
the later "Memoirs" by J. W. Powell, 1875, and C. E. Dutton,
1882.
CHAPTEE IX.
FLUVIATIL® AGENCIES.
2.—Action of Glaciers.
Formation of Glaciers.—On iigli mountains
and in high latitudes, as in Greenland or Lapland,
the water precipitated from the atmosphere falls as snow
instead of rain. This occui-s even in the Torrid Zone wher¬
ever the land exceeds an altitude of 15,000 or 16,000 feet
above the sea, and in all other latitudes at a less height in
proportion to their distance from the Ton-id Zone. In the
Alps the snow-line in summer occurs at a height of 8,800
feet ahove the sea. Near the North and South Poles it
comes down to the level of the sea. In regions of perpetual
snow even the hottest summer's sun fails to melt all the
snow that falls in a year, hence there is a continual accu¬
mulation of it, and, if there were no way of getting rid of
it, aU the water in the world would be eventually trans¬
ferred to these regions and remain there in a solid form.
There is, however, a drainage from these snow-covered
regions by means of glaciers, just as there is a drainage
from other regions by means of brooks and rivers. The
lower part of all perpetual snow is being constantly con¬
verted into ice ; the weight of the superincumbent mass
forces out the air from between the snow-flakes and binds
them together, just as a snowball is made harder by the
pressure of the hand. This firm, granular kind of snow is
known as névé or firn, and as it is pushed down the moun¬
tain slopes it passes into a kind of soft ice, partly in con¬
sequence of the pressure from above, and partly from the
alternate melting and freezing of the surface layers : the
152
dynamical geology.
[sec. ii.
sun melts the surface in the daytime, and the water so
formed percolates down into the mass and becomes frozen
again below.
The ice thus formed is gradually pushed down into the
■ valleys, along which it continues to travel in the form of
ice-rivers or glaciers. The glaciers of the Alps are some¬
times as much as 600 feet in depth, and they fill the
valleys for many miles below the snow-line, till they come
down to a level of about 3,600 feet below it, where they
terminate, in consequence of the ice melting into water and
proceeding as a brook or a river. In the New Zealand Alps,
where the summer snow-line is at about 5,000 feet, the
glaciers on the Mount Cook range, near lat. 44, descend to
about 2,300 feet on the eastern side, and one on the western
side comes down to within 670 feet of the sea, discharging
its load of débris in the midst of a sub-tropical vegetation.'
In high latitudes, where the snow-line is near the sea-
level, the ice is pushed into the sea, and the portions
which break off from time to time are floated off as
icebergs.
Movement of Glaciers.—Glaciers creep along their
beds at a rate which varies with the supply of ice, the
slope of the valley, and the season of the year, but their
movement is seldom so rapid as to be perceptible without
careful measurement. Like a river, the ice moves faster in
the centre than near the sides, and the surface layers move
faster than the lower portions, and the whole moves as if it
were a plastic or viscous body. The lower parts of the
Swiss glaciers, where the gradient is not steep, only move
at the rate of one or two feet in 24 hours ; thus the Mer de
Glace in summer moves at an average rate of 27 inches a
day in the centre, and from 13 to 19 inches a day near the
sides ; in the winter the rates are only about half as much.
Where slopes are very steep ice-falls are formed, the ice
descending in huge steps or slices, with long fissures or
crevasses between, but on reaching a gentler slope the
fissures close and are frozen together again. Such ice-falls
are frequent in Norway.
Where there are large snow-fields, and the glaciers pass
' Green's " High Alps of New Zealand," 1883, p. 75.
Fig. 38. Ideal representation of a tilaeier.
154
dynamical geology.
[sec. 11.
down fairly steep slopes, their movement is much faster
than any of the Swiss glaciers. Thus at G-lacier Bay, in
Alaska, nine large and seventeen small glaciers unite to
form an ice-stream which has a front of 5,000 feet in
width and a depth of 700 feet where it enters the bay :
this stream moves during August at the rate of 70 feet a
day in the middle, and 10 feet near the sides.
Transport by Glaciers.—We have seen that, in the
case of rivers, two processes are going on at the same time,
namely, erosion and transportation ; the rivers of ice per¬
form the same kind of work, but in a different way.
Wherever a glacier passes beneath a crag or cliff of rock it
receives on to its surface a continued supply of blocks
and stones which are detached by the action of frost and
weathering, described on p. 102. A line of such rock-frag¬
ments may generally be seen at each side of the glacier.
Where two glacier-streams unite the two lines of tran¬
sported stones on their adjacent sides come together and
form a double median line along the centre of the glacier
below the junction of the two valleys. Thus, if a glacier
happen to have many important tributaries proceeding
from different valleys, and uniting to form one large ice-
stream, this may come to have many separate lines of stones
and rubbish, which are all carried along to the place where
the glacier terminates. Here they are all thrown down,
and form an irregular mound or pile of rough, angular
debris mixed with earth, which in Switzerland is called a
moraine. This term is also applied to the lines of stones
on the glacier, so that there are three kinds of moraines,
lateral, medial, and terminal. (See fig. 38.)
Very large blocks of rock are sometimes transported by
glaciers to great distances from their parent sources, and in
those countries where the glaciers were once larger and
longer than they are now, such blocks are found scattered
over the area formerly covered by the ice ; they are often
perched on eminences, or left stranded on the sides of a
valley. Rock-masses found in such positions are called
perched blochs or erratics. (See fig. 39.)
Some of the clearest and best-known instances of the
transport of such blocks are found in Switzerland. There
was a time, generally known as the Glacial Period, when
CHAP. IX.J
FLUVIATILE AGENCIES.
155
the Swiss glaciers, instead of ending as they do now, high
up in the valleys, nearly filled the valleys, and coalescing
in their lower parts spread far out on the plains below.
Fig. 40 shows the former extent of some of these glaciers,
and it will be seen that the old glacier of the Rhone
stretched right across the plains of Geneva and Neuchatel,
so as to rest against the flanks of the Jura. The height
to which the ice then reached on the Jura is marked by a
long line or belt of boulders, extending along the flanks of
Fig. 39. View of a Perched Block.
these mountains at an average height of 800 feet above the
lake of Neuchatel. The rocks of the Jura are chiefly lime¬
stone and shale, while the blocks consist of granite gneiss
and other crystalline rocks which only occur in the Alps.
Scattered blocks of similar rocks are traceable all down the
valley of the Rhone to Lake Geneva, and it is noticeable that
those on one side the valley have been brought from moun¬
tains higher up on the same side, that they are in fact the
remnants of the lateral moraine on that side of the old
glacier. Thus the proof of their having been transported
156
dynamical geology.
[sec.
II.
by a glacier is complete. Some of these blocks are
large as houses, and weigh 2,000 or 3,000 tons.
as
o
S ^
«
a
es
S
be
S
<1;
Erosive Action.—The surface of a glacier is not always
smooth ; on the contrary, it is generally rough and broken,
CHAP. IX.J
FLUVIAÏILE AGENCIES.
157
and sometimes it is traversed by deep cracks or clefts
called crevasses. Many stones and blocks fall down these,
others are picked up from the bed of the glacier and get
firmly frozen into the ice. The stones thus fixed in the
under surface of a glacier scratch and groove and wear
away the rocks over which they are moved, and are also
scratched and worn themselves by the hard projections of
the rocks beneath. One result of this action is the produc¬
tion of a quantity of fine mud and sand, so that if a lump
of glacier ice is dissolved in a glass of water, it makes the
water as turbid as if a spoonful of milk had been put into
it. The river that always springs from the end of a glacier
carries away all this mud, and flows as a dirty yellowish or
greenish white stream, until it reaches the sea or some
great lake in which the sediment may be deposited.
It must not be assumed, however, that all the mud in
the stream which issues from the end of a glacier is the
result of erosion by the ice. Some of it is earth which has
fallen on to the ice with the stones of the moraines, and in
summer there are always streams of water which course
over the surface of a glacier till they plunge into a cre¬
vasse ; frequently also springs issue from the rocks below
the ice. In this way a sub-glacial river comes to be formed,
which flows beneath the ice, and acts hke other rivers in
eroding its channel.
One marked and characteristic result of the grinding and
rasping action exercised by a large glacier is the wearing
and rounding off of all the prominences over which it
passes, and a general smoothening and polishing of the
rocky sm'faces, so as to produce a peculiarly moulded out¬
line, which is quite different from the surface features pro¬
duced by any other agency. If we examine a valley where
the glacier once extended much further down than it does
now, we shall find that the ice has left a smoothened sur¬
face of rock, rising here and there into rounded bosses or
hummocks, which have a smooth, polished, and gently
sloping surface on one side, with a rougher and steeper
slope on the other ; we shall also notice that the smoother
faces of these hummocks all look up the valley towards the
quarter whence the moving ice came. When viewed from
a distance they have been thought to bear some resem-
158
DYNAMICAL GEOLOGY.
[sec. II.
blance to a flock of sheep lying down on the ground, and
have hence been called roches moutonnées.
Passing down the valley we may also come upon the
terminal moraine of the ancient glacier, still remaining as
a mound of rubbish and rock debris stretching across the
valley in a semicircular or horseshoe-shaped curve.
It is not only in countries where glaciers now exist that
these ice-marks are fouud. Many valleys in the north of
England, in Scotland, Ireland, and other parts of the
world, present the same appearances as those of Switzer¬
land, only the tool-marks are not quite so fresh-looking.
The rounded forms of the roches moutonnées are, however,
just as remarkable, and in some valleys a series of terminal
Fig. 41. Roches Moutonnées in South Wales.
Reproduced by permission from " Quart. Journ. Geol. Soc.,"
vol. xxxix. p. 50.
moraines may be found, one above another, showing how
the glacier gradually dwindled away, and shrank higher
and higher up the valley.
From the above description it is clear that the passage of
glacier ice down a valley will scrape out all the soils and
superficial deposits that may previously have accumulated
in the valley, and will sweep the materials onward to swell
its terminal moraine. Any hollows or depressions which
may have existed in the valley, and were, perhaps, filled
up with alluvial deposits before the glacier was developed,
will be cleared out by the ice, and subsequently, when
the glacier has disappeared, these hollows will remain as
lakes.
CHAP. IX.J
FLUVIATILE AGENCIES.
159
It has eyen been suggested by Sir A. Eamsay that the
rock basins in which many lakes lie have actually been
excavated by the erosive action of the glaciers ; on this
point, however, there is much difference of opinion among
geologists ; and none of the facts brought forward by
Eamsay in support of his view can be regarded as proving
its truth. Thus it is a fact that lakes are most numerous
in those countries which are known to have been swept by
the ice of the G-lacial Period ; but the rock basins in which
these lakes lie may have been pre-existent, and have simply
been cleared out by the glaciers in the manner just de¬
scribed. The fact that glacial strise can be traced through
these rock basins is no proof that the ice made the hol¬
lows. Lastly, glaciers probably have most erosive power
where the ice is forced through narrow defiles and down
steep slopes, and not, as Eamsay imagined, where they
impinge on gentle slopes or plains.
Actual observations seem to show that where a glacier
reaches a level space it does not exercise any erosive power,
but, if it is advancing, it may override its own moraine,
without ploughing through it. Professor J. W. Spencer has
described several glaciers in Norway, which, though ad¬
vancing, are not doing any erosive work. He found that
a vCTy slight ridge of rock was suflScient to divert the flow
of the ice ; and that large stones, instead of being held
fast, and used as grooving tools, often left a groove or
furrow in the ice which had passed over them. It must be
remembered, however, that the erosive power of a glacier
must be greatest where the ice is thickest and the rate of
movement greatest, whereas at its terminal extremity the
ice is thin and the movement slow.
Ice Sheets.—In Arctic and Antarctic regions, where pre¬
cipitation is always in the form of snow, and the summer
sun never melts all the winter snow-fall, the snow accumu¬
lates to such an extent as to cover large areas of land, and
to form massive ice-sheets which may be several thousand
feet thick.
Greenland is covered by such a mantle of ice and snow,
and this has recently been traversed from one coast to the
other by Dr. Nansen, who thus describes it ; " The ice-
sheet rises comparatively abruptly from the sea on both
160
dynamical geology.
[sec. II.
sides, but more especially on the east coast, while its
central portion is nearly flat. On the whole the gradient
decreases the further one gets into the interior, and the
mass thus presents the form of a shield, with a surface
corrugated by gentle, almost imperceptible, undulations,
lying more or less north and south." The highest part is
about 9,000 feet above the sea, and rather nearer the east
coast than the west. How far this ice-sheet extends
toward the north is not yet known ; but Dr. Nansen re¬
marks that its limit must lie beyond the 75th parallel, for
so far along the west coast it sends huge glacier arms into
the sea. Among these is Upemivik Glacier, which moves
down at the rate of 99 feet in twenty-four hours. The
thickness of this ice-sheet in the interior cannot, of course,
be estimated ; but no mountains rise above it, and those
parts of it which come near the coast are considered to be
from 2,000 to 3,000 feet thick.
On the western side of the country the edge of the inland
ice is from 50 to 100 miles from the coast ; but on the east
coast large portions of it descend into the sea, and the peaks
of nearly buried mountains (nunutaks) can be seen project¬
ing above its surface, and only small tracts near the shore
are free from ice. Both coasts are indented by fiords, the
upper ends of which are generally barred by the ice-cliffs
in which the huge glaciers terminate, and from which ice¬
bergs are being continually given off.
Nansen gives a section across South Greenland which
shows the surface contour of the inland ice, but he makes
no attempt to indicate the form of the ground beneath,
though his observations afford some data for doing so.
Thus the mountain peaks which rise through the ice
near the east coast were all from 3,000 to 4,000 feet high,
and between 40 and 50 miles inland he saw several nunu¬
taks with elevations of over 6,000 feet. On the west coast
the highest ground is often near the sea, and away from
the inland ice, though in places this is parted by peaks and
ridges from 2,000 to 3,000 feet high. The diag:ram, fig. 42,
has been drawn in accordance with these indications, and
will at any rate serve to give some idea of an ice-buried
country. Pig. 43 is a portion of the inland ice descending
to the sea as a glacier.
West coast.
Near Gotthaab.
East coast.
Fig. 42. Diagrammatic Section through the Ice-sheet of Southern Greenland.
Horizontal scale about 50 miles to an inch.
Fig. 43. View of a Glacier descending from the Inland Ice of Greenland.
162
DYNAMICAL GEOLOGY.
[sec. ii.
The interior of the island of Spitzbergen is also covered
with a snow-field and ice-sheet from which huge glaciers
descend into the sea ; indeed, some parts of the coast con¬
sist almost entirely of the ice-cliffs in which the glaciers
terminate. Mr. J. Lament has described a glacier on the
south-east coast which has a continuous front of ice 30
miles wide, and projects into the sea in three great semi¬
circular divisions, the largest of them protruding for three
or four miles beyond the real coast-line. The frontal ice-
cliffs of this glacier vary from 20 to 100 feet in height, and
from them masses of all sizes—up to that of a church—are
continually breaking off and crashing into the sea. The
two outer lobes consist of smooth ice, but the inner one is
so rough and jagged that, when seen dimly through the
fog, " it resembles more than anything else a forest of pine-
trees covered with snow."
The terminal position of some of these glaciers appears
to vary; some have advanced and others retreated since
they were first observed. Nordenskiold states that in
BeU's Sound there was in 1858 a harbour, at the head of
which was a strip of lowland, and beyond this a low but
broad glacier. In 1860-61 the glacier advanced over the
lowland, filled up the harbour, and extended far into the
sea. " It now constitutes one of the largest glaciers in
Spitzbergen, from which immense blocks of ice constantly
fall down, so that not even a boat can venture in safety
beneath its broken border." '
Mr. Lament says that many of the bergs which floated
away from the ice-cliffs of Spitzbergen "were heavily
charged with clay and stones," and that the sea for miles
around is sometimes discoloured from the quantity of mud
which is washed off this floating land-ice by the waves,
and brought down by the torrents which elsewhere descend
from the mountains.
Where mountains rise through an ice-sheet, lines of
surface moraine are found, but when a whole region is
buried in ice there can of course be no surface moraines,
and the transport of detritus must be confined to the sole
or base of the ice-sheet, and to the sub-current rivers. With
' " Geol. Mag." Dec. 2, vol. ill. p. 18.
CHAP. IX.J FLUVIATILE AGENCIES.
163
respect to the capacity of such ice-sheets for transport and
erosion we know very little at present from actual obser¬
vation, but as regards erosion, it may be safely asserted
that if any part of a valley glacier exercises erosive power,
those parts of an ice-sheet which move down buried valleys
must have a more powerful erosive action.
It is believed that, at the period when the Swiss glaciers
attained their greatest development and extension. North
Britain and Scandinavia were covered by ice-sheets which
moved out in every direction from the main lines of water¬
shed, which were then great gathering grounds of snow.
The smoothened and rounded outlines of the lower hills in
these countries, and the scratched and polished surfaces of
the rocks on their sides, are generally attributed to the
rasping and grinding action of these enormous masses of
ice.
CHAPTEE X.
MÂBINE AGENCIES.
CTION of Waves.—The upper surface of the sea is
kept in continual agitation by the force of the winds,
which raise it into waves of all dimensions ; these waves roU
in upon every exposed coast-line throughout the world, and
act as instruments of erosion and destruction. To estimate
the power of sea-waves, they must be seen under the in¬
fluence of a storm or gale of wind. Just as a brook in
time of drought furnishes no measure of the work it does in
time of flood, so the sea on a calm day gives no idea of the
force with which storm waves can break on a rocky shore.
It has been ascertained that the average force of the
breakers in winter time on the west coast of Scotland is
equal to a pressure of 2,086 lbs. (nearly a ton) on every
square foot of rock ; during a storm it is greater. Mr.
Stevenson found by experiments at the Bell Eock Light¬
house that the pressure was sometimes a ton and a half
upon the square foot, and that at Skerryvore in the
Atlantic it was doubly powerful, viz., about three tons to
the square foot.
Such a force is evidently capable of dislodging and
moving very large fragments of rock, especially if the
blocks have been previously loosened by the action of rain
and frost. Sir Arch. Geikie ^ states that on the coast of
the Pentland Firth blocks from 7 to 13 tons in weight had
been quarried out of the cliffs at a height of 70 feet above
the sea ; the waves must have risen to that height, and
still have been able to bring down these immense frag-
* " Scenery and Geology of Scotland,'' p. 61.
CHAP. X.J
MARINE AGENCIES.
165
ments. In other places they have torn up similar masses
of rock, and heaped them together at a height of 62 feet.
The damage and destruction which is often wrought
upon piers and breakwaters during storms enables us to
realize the enormous power of the waves, and anyone who
has witnessed the making of a breach in such massive
masonry will be better able to appreciate the action of the
sea upon coasts and cliffs. During heavy gales at Ply¬
mouth in 1824 and 1829, blocks of limestone and granite,
weighing from to two to five tons, were washed about at the
breakwater like pebbles. About 800 tons of such blocks
were borne a distance of 200 feet up the inclined plane of
the breakwater. In one place a block of limestone, seven
tons in weight, was washed a distance of 150 feet.^ On the
coast of Clare, in Ireland, blocks of rock may be seen
which have been torn from rock-masses 20 feet above high-
water mark, and thrown up on to the grass full 20 feet
higher and 20 yards farther back.^
The mere blow and force of the wave, however, is not
the whole of the force exercised ; when the water is dashed
against a rock or cliff, some of it is injected into every
crack and crevice it can find, and its pressure is exerted in
forcing asunder the walls of the fissure. Moreover the
air that was previously in the fissures is suddenly com¬
pressed and impelled into the minuter cracks and planes of
division. With the recess of the wave the pressure is
suddenly relieved, and the air and water rush out with a
suction that is capable of loosening and dislodging large
blocks of rock. Ño sooner are such fragments torn away
from the cliff than they are immediately converted into
engines of further destruction, for they are lifted by the
waves and hurled against the rocks, so as to act like ham¬
mers or battering rams.
To sum up, therefore, it may be said that when sea-
waves break full against the face of a cliff, four different
physical processes are set in action, all of which assist in
the work of destruction : these processes are :—
1. The force and impact of the waves.
' De la Beche's " Geological Observer,'' p. 47.
^ Jukes' "Manual of Geology," second edition, p. 220.
166
dynamical geology.
[sec. n.
2. The hydraulic pressure of the water in the larger
fissures.
3. The alternate compression and expansion of air in the
cracks of the rock.
4. The employment of stones and boulders as battering-
rams.
Formation of Caves, Coves, and Inlets.—If the
larger cracks and fissures run at right angles to the cliff,
they form so many lines of weakness, which yield more
readily to the continued action of the waves, and a long
cave or else an open inlet is gradually formed. In other
cases the larger cracks or master joints run parallel to the
face of the cliff ; shorter and wider cavities are then
formed near the base of the cliff, which is thus undermined
till one of the main joints is reached, when the unsupported
mass above falls in ruin on the shore. The breakers then
proceed to reduce the size of the fallen fragments, dashing
them one against the other, and breaking them up into
pebbles and sand, till at last they are washed away, and
the face of the cliff is exposed again to fresh assaults.
Instances of the action of the breakers on jointed rocks
are to be seen on all coasts. On the western coasts of
Europe the severest storms and strongest gales come from
the westward, with winds ranging from S.W. to N.W.,and
as they sweep over the vast expanse of the Atlantic Ocean
they raise up enormous waves which break upon the land
with tremendous force. The hard rocks of the western
coast of Ireland afford many illustrative examples of the
action of these breakers as going on at present, their cliffs,
caves, and rocky islets having been formed by inroads.
Mr. Jukes states, on the authority of Mr. W. L. Willson,
late of the Geological Survey of Ireland, that in the far
part of the promontory between Bantry and Dunmanus
Bays there are dark holes in the fields some distance back
from the edge of the cliffs, looking down into which the
sea might be dimly seen washing backwards and forwards
in the narrow cavern below.
" At high water, and during gales of wind, with heavy-
breakers rolling in upon the coast, vast volumes of water
are poured suddenly into these narrow caverns, and rolling
on, compress the air at their further end into every joint
CHAP. X.J
MARINE AGENCIES.
167
and pore of the rock above, and then, suddenly receding,
suck both air and water back again with such force as now
and then to loosen some part of the roof. Working in this
way, the sea sometimes gradually forms a passage for itself
to the surface above, and if that be not too lofty, forms a
'blow-hole,' or 'puffing-hole,' through which spouts of
foam and spray are occasionally ejected high into the
air." ^
Some of these blow-holes open down into cavernous
gullies, which lead from one cove to another, behind bold
headlands of even a hundred or more feet in height ; show¬
ing the commencement of the process by which headlands
are converted in islands. One such square precipitous
island, near Loop Head, County Clare—which in 1862 was
miles), a, Mupe Cove, b, Lulworth Cove, c. Stare Cove.
d, Man-of-War Cove, e, Durdle Cove, f, Barn Door.
at least 20 yards from the mainland—was said by the
farmer who held the ground to have been accessible by a
twelve-foot plank when he was a boy.
Another cause of the irregularities of a coast-line is to
be found in the unequal hardness or resistant power of the
rocks themselves. 'The more yielding strata are worn into
deep bays and inlets, while the harder rocks stand out as
capes, promontories, and detached islets.
Nowhere is this result so clearly seen as on the south¬
west coast of Ireland, where beds of indurated sandstone
form the bold headlands separating Dingle, Kenmare,
Bantry, and Dunmanus Bays, while these inlets are all
eroded out of more destructible shales and limestones.
The coasts of Devon and Cornwall present similar
' Jukes' " Manual of Geology," second edition, p. 220.
168
DYNAMICAL GEOLOGY,
[sec. ii,
examples on a smaller scale, the numerous creeks and
coves being all scooped out of some comparatively soft
rock, frequently the shaly slate locally known as killas,
while the harder rocks, such as granite and dolerite, have
offered more resistance to the breakers and stand out
as headlands and promontories. Polventon Cove, on the
east side of Trevose Head, is a natural harbour formed in
this way, and protected from the north-west winds by a
jutting ridge of hard, igneous rock (greenstone), which
forms an efficient breakwater.' The erosion of the shores
of Tor Bay has been described by Mr. Pengelly.'
In England, however, the most remarkable instances of
this selective action of the waves are to be found on the
Dorsetshire coast. The cliffs which form this coast con¬
sist of a variety of strata, some of which are very much
softer and more yielding than the rest. As a consequence,
wherever these are brought within the reach of the waves
deep recesses, or coves, have been excavated out of them.
No less than six of these coves occur within the distance of
five miles, while the intervening portions of the coast-Hne
form nearly straight ranges of cliffs. These features are
shown in plan in fig. 44, and an idea of the scenery they
produce may be obtained from the view of Durdle Cove in
fig. 45 ; the eastern cape of this bay is formed by a pro¬
jecting crag of hard limestone (Portland stone), through
which the waves have worn a wide gap or archway known
as the " Barn-door." The western cape consists of chalk,
while the inner cliffs consist chiefly of sands and clays, to
the presence of which the bay owes its existence.
The Isle of Wight owes its lozenge shape entirely to the
relative hardness of the rocks of which it is composed.
Through the middle of the island runs a high ridge of
chalk, the two ends of which project east and west, and
form the promontories known respectively as Culver Point
and The Needles (see fig. 46, borrowed from Mantell's
" Excursions "). The Cnlver cliffs are flanked by Sandown
and Whitecliff Bays, while Brixton and Alum Bays occupy
the same position with regard to the Needles. All these
' See De la Beehe's " Geological Observer," p. 51.
^ "Geologist," vol. iv. p. 447.
Fig. 45. Durdle Cove, seen from the east end. (From Mantell's " Geological Excursions.")
170
DYNAMICAL GEOLOGY.
[sec. ii.
bays have been hollowed out of the softer sands and
clays which lie on either side of the chalk ridge ; and we
may further observe that the deeper hollows of Sandown
and Brixton Bays are doubtless due to the fact that their
coast-lines waste more rapidly than do those of the more
northern bays.
Fig. 46. The Xeedles, from Headon Hill, looking southward.
Formation of Cliffs,—The foregoing facts and descrip¬
tions demonstrate the powerful action of the sea on the
land; we have now to learn the limits which are set to
this action.
The ordinary action of the waves is confined to the space
between high and low water mark, and their force is found
to be greatest above half-tide level and before the height of
the tide. Moreover it is the upper or plunging part of the
wave which possesses erosive power, the lower waters being
accumulative in their action, first pushing onward and
then withdrawing the pebbles of the beach. As the tide
recedes frcm half-tide level, the surges become smaller and
smaller, and the backward current stronger, till at low tide
chap. x.]
marine agencies.
171
the sea is comparatively quiet. Between these points, there¬
fore, a line is reached where the erosive and accumulative
actions balance one another, and below which little erosion
takes place. This line is always a little above the level of
low tide, and may be called the line of least erosion.
During storms, however, the power of the waves is still
great even at low tide, and they disturb the bottom to
some depth below the low water level, as is testified by the
shells, stones, and seaweeds torn from their habitat below
low water mark and cast up on the beach after storms.
During violent gales it is said that the sea-floor has been
disturbed to a depth of 50 fathoms, but such disturbance
would only cause an oscillation of the sand and mud at the
bottom.
On a rocky shore the tendency of these conditions is to
produce a horizontal or gently sloping platform, the outer
edge of which corresponds to the line of non-erosion, so
that its surface is bare at low water. This platform is often
called a scar, and ,its inner side is bounded by cliffs more
or less lofty, according to the height of the land which is
eaten into. Pig. 47 is a section across such a platform,
the line a representing high tide level, and b low tide level.
When the land was first brought into the position with
regard to the sea-level which it is supposed to occupy in
fig. 47, no chffs existed, but the line e p was a continuous
slope. The sea began to cut into the 'land at the point e,
and the formation of the platform has been effected by the
recession of the cliffs to the point e.
The rapidity of Marine Erosion.—The rate at which
cliffs are being worn back depends on various circumstances,
partly on the position of the cliff and depth of water at
A
B
Fig. 47. Formation of a Shore Platform.
172
dynamical geology.
[sec. 11.
its base, partly on the force of the breakers, partly on the
hardness of the rocks composing the cliff, and partly on
the assistance derived from sub-aërial agencies—rain, frost,
and springs—which generally work faster than the waves,
and cause the cliffs to slope backward at a greater or less
angle from the shore.
The most important condition is the hardness or re¬
sistant power of the rocks themselves. Where the rocks
of the coast are hard and massive little change is noticed
in the course of a century, but where the whole coast con¬
sists of the softer kinds of rocks, and especially if it is
swept by a strong current which can readily carry off the
eroded materials, the waste of land is sometimes very rapid,
so that all the inhabitants become sensible of it. This is
the case for long distances on the eastern and southern
coasts of England, and even within the last few centuries
the destniction of land has been so great, that the sites of
many villages and some considerable towns along the
coasts of Yorkshire, Norfolk, Suffolk, Essex, and Kent,
are now beneath the sea, which has eaten away the groimd
on which they once stood. It should be observed, how¬
ever, that it is only at certain points along these coasts
that destruction is going on, not along their whole extent.
At some places, as will hereafter be mentioned, deposition
and accretion are taking place. For the facts which follow,
my authority, when another is not specially cited, is the
tenth edition of Lyell's " Principles of Geology."
In Yorkshire the waste is most rapid along the coast of
Holderness, between Flamborough Head and Spurn Point,
at the entrance of the Humber ; this tract consists of beds
of clay, gravel, and sand, which form low cliffs against
which the waves beat, while a strong current sets from
the north and carries away the debris.
" For many years," says Professor Phillips, " the rate
at which the cliffs recede from Bridlington to Spurn, a
distance of 36 miles, has been found by measurement to
equal on an average 2i yards annually, which upon 36
miles of coast would amount to about 30 acres a year. At
this rate the coast, the mean height of which above the
sea is about 40 feet, has lost one mile in breadth since
the Norman Conquest, and more than two miles since
CHAP. X.J
MAEINE AGENCIES.
173
the occupation of York by the Romans." In some places
the loss of land is said to be as much as 5 yards per
annum. ^
The sites of the Tillages of Auburn, Hartburn, Hyde,
and EaTenspur, are now sand-banks dry only at low
water, though Eavenspur, where Bolingbroke landed to
depose Richard II., was once so large and flourishing a
sea-port as to be a rival of Hull.
Norfolk.—The site of the old town of Cromer is now in
the G-erman Ocean, the inhabitants having continually
built inland as the sea gained on them. It is stated by
Mr. Redman ' that during the twenty-three years which
elapsed between the Ordnance Survey of 1838 and the
year 1861, a portion of the cliff composed of sand and
clay between Cromer and Mundesley receded 330 feet,
amounting to a mean annual waste of 14 feet ; these cliffs
are from 200 to 250 feet high, and the amount of material
carried away from them every year must, therefore, be
enormous. The facts given by the same authority show
that along the cliffs between Mundesley and Happisburgh
the average annual rate of waste is about three yards.
This waste is largely due to the work of rain and springs,
for the cliffs often founder down in a series of landslips ;
but this movement would soon cease if it were not for the
removal of the talus by the action of the sea.
On the same coast churches, villages, and manors, such
as those of Shipden, Wimpwell, and Eccles, have one after
another disappeared, so that their previous existence is
only known from old records.
Suffolk has suffered in a similar manner. Dunwich,
now only an inconsiderable village, was once the chief port
in the county. It is stated in Doomsday Book that two
tracts of land outside the village had been taxed by
Edward the Confessor, but had been destroyed in the few
years which had elapsed between that time and the
Conqueror's Survey. In other later records mention is
made of, at one time, a monastery, at another several
' See a paper " On the Encroachments of the Sea from Spurn
Point to Flamborough Head," by R. Pickwell, " Proc. Inst. Civ.
Eng." vol. li. p. 191.
® " Proc. Inst. Civ. Eng.'' vol. xxiii. p. 31.
174
DYNAMICAL GEOLOGY.
[sec. II.
churches, then the old port, then 400 houses at once, and
gradually the gaol, the town-hall, the high roads, then of
ancient cemeteries, the coffins of which were for some time
exposed in the cliff—all swept away by the devouring sea.
The waste of Dunwich cliff has ceased for some years,
and shingle now accumulates at its base ; but the neigh¬
bouring cliffs of Easton Bavent and Covehithe are re¬
ceding at the rate of from 20 to 30 feet in a year.'
Beacon Cliff, near Harwich, has wasted at a very rapid
rate ; it is about 50 feet high, and consists entirely of clay.
Captain Washington ascertidned that between the years
1709 and 1766 it had receded a distance of 40 feet; be¬
tween 1756 and 1804, 80 feet had been swept away, and
between 1804 and 1841, no less than 350 feet, showing a
rapidly accelerated rate of destruction.
Kent.—The Isle of Sheppey, off the north coast, furnishes
another example of the rapid destruction of clay cliffs,
which are continually foundering down under the combined
attacks of rain-water and sea-waves. An account of the
landslips constantly taking place along the north coast of
this island has already been given (p. 121), and the sea is
chiefly employed in washing away the ruins which fall on
the shore. As much as 50 acres of land have been lost
in 20 years, and if the present rate of destruction should
continue, it is easy to foresee the total destruction of the
island.
Still farther east stands the church of Eeculver, upon a
low cliff of clay and sand ; Eeculver was a Eoman station,
and even in Henry VIII.'s time the church was nearly a
mile from the sea. The Eoman camp was destroyed in
1780, and part of the churchyard in 1804, but, since then,
artificial means have been taken to break the force of the
waves and to preserve the church as a landmark.'
Sussex.—^Westward of Hastings there has been much
loss of land, and though Pevensey Level, with its border of
shingle, has been formed in historical times, yet for more
than a century the whole shore-line has been giving way,
the rate of waste now being between 3 and 7 feet per
^ See "The Geology of Southwold," by W. Whitaker, Mem.
Geol. Surv. 1887.
» See Lyell's " Principles," vol. i. p. 523.
chap. x.]
MARINE AGENCIES.
175
annum. At Langney Point between the years 1736 (Des-
marest's Survey) and 1844 (Ordnance Survey) the sea had
advanced more than half a mile. A comparison between
the two Surveys above mentioned shows that the coast
between Eastbourne and Beachy Head has receded to the
extent of more than a quarter of a mile, and in some places
as much as 600 yards, the average loss being between 12
and 15 feet per annum ^ (see map, fig. 48). Of the mar-
tello towers constructed here in 1806, four (Nos. 69 to 72)
have since been destroyed by the encroachments of the sea,
and are now below highwater mark.
At Beachy Head, which is a promontory of hard chalk,
the erosion is less rapid, but falls from the cliff are fre¬
quent, and the largest landslip of which record remains
took place in 1813, when a mass of chalk 300 feet long
and about 80 feet wide was precipitated on to the shore.''
Seven towers of chalk called the Seven Charles formerly
stood out from these cliffs, but the last of them, which was
known to have withstood the attacks of the sea for more
than a century, fell in 1853.®
Between Newhaven and Brighton the average rate of
waste is 3 feet per annum, and Mr. H. Willett states that
at Aldrington (3 miles west of Brighton) the encroach¬
ment of the sea amounted to 270 feet in ten years, or an
annual rate of 9 yards, these figures being obtained by
actual admeasurement.
The promontory known as Selsey Bill has likewise suf¬
fered great loss. Im the time of the historian Bede, a.d. 731,
Selsey was a peninsula, and contained 5,220 acres of land ;
at the present time its area only amounts to 2,880 acres.
It was for many years a Bishop's see, and possessed a
cathedral and episcopal palace with a large park, which in
the time of Henry VIII. was well stocked with deer. Now
the site of these buildings is said to be about a mile from
the coast, and a small plot called Park Coppice is the
only remnant of the park ; though the shore and sands
' Bedman, " Proc. Inst. Civil Engineers," vol. xxiii. For the
use of this illustration (fig. 48) I am indebted to Rev. H. E.
Maddock and Mr. Chambers, of Eastbourne.
" Webster, " Trans. Geol. Soc." vol. ii. p. 191.
' Chambers' ' ' Guide to Eastbourne. "
Fig. 48. CMianges in the coast-line between Beachy Head and Pevensey. The shaded parts are land,
the lined portion consisting of sand and shingle. The numbers indicate martello towers.
Scale about one mile to an inch.
CHAP. X.J
MARINE AGENCIES.
177
are still known locally by the name of the " Bishop's
Park."
Hampshire.—The cliffs between Hurst Shingle Bar and
Christchurch are likewise worn back at a rapid rate, the
destruction being estimated at a yard per annum. Christ-
church Head stands out as a promontory, and it is a note¬
worthy and significant fact that this is the only point
between Lymington and Poole Harbour in Dorsetshire
where any hard stony masses occur in the cliffs. " Five
layers of large ferruginous concretions," says LyeU, " some¬
what like- the septaria of the London Clay, have occasioned
a resistance at this point, to which we may ascribe this
headland." '
The configuration of the Isle of Wight, and the extent
to which it has suffered from marine erosion, have already
been mentioned (see p. 168).
Dorsetshire.—The same phenomena are repeated in the
districts of Purbeck and Portland. The manner in which
the coves of the Purbeck coast have been formed has
already been mentioned ; at Portland the erosion has
been much more considerable : the cliffs here consist of a
soft, shaly clay, overlaid by a massive limestone ; the clay
is rapidly excavated, so that the cliffs are undermined, and
large masses of the superstratum then fall by their own
weight. The action of the waves is assisted by that of the
springs thrown out beneath the limestone, as in the Isle of
Wight, p. 123. Large landslips have often occurred, notably
one in 1792, when a tract of ground a mile and a quarter
long and 600 yards broad was moved towards the sea. The
isle of Portland is, in fact, the last remnant of a once ex¬
tensive tract of land.
Similar destruction and loss of land is going on along
the coast between Bridport and Axmouth, the great land¬
slip near the latter place having been already mentioned
(p. 124). Near Lyme Eegis the rate of waste has been
estimated at about three feet per annum.
Erosion of Heligoland.—The destruction of land
which can be accomplished by the sea under certain circum¬
stances will be better realized if we take the case of a
^ " Principles of Geology," tenth edition, vol. i. p. 533.
N
178
DYNAMICAL GEOLOGY.
[sec. ii.
smaller island than England. Heligoland is an island in
the North Sea about forty miles west of Holstein. It
consists of materials similar to those which form the east
Fig. 49. The erosion of Heligoland. Scale about 10 miles to an
inch. The outermost line represents the outline of the island
in a.d. 800 when 120 miles in circumference. The area shaded
by broken lines is that of the island in a.d. 1300, when it was
about 4.5 mUes in circumference. The smaller shaded areas are
the two islands which existed in 1649, the larger being then
only 8 miles round.
Fin-, 50. A Bird's-eye View of Heligoland from the south-east.
180
DYNAMICAL GEOLOGY.
[sec. 11.
of England. No accurate description exists of its size in
very early times, but it is known to have been continually
diminished by incursions of the sea during high tides and
storms. Fig. 49 is a reduced copy of a map said to have
been " copied from an old map in possession of the
Governor of Heligoland," and showing the area of the
island at three different periods.
The outlines here shown may not be strictly accurate,
and the area represented as existing in a.D. 300 may pos¬
sibly be rather too large,^ but the map may be taken to
show the approximate size and the relative position of the
portions destroyed between the dates mentioned. It is
very probable that the larger part of the island, as it
existed in a.D. 800, was low and flat, hence the great loss
of land between that date and 1300. There can be little
doubt that the portion now remaining has survived be¬
cause it was the highest and firmest part of the island.
It is now about a mile long and two furlongs broad at its
widest part, with cliffs 170 feet high (see fig. 50). About
half a mile eastward is Sandy Island, which was united to
the main island by a ridge of chalk till as late as 1720,
but having been largely quarried by the inhabitants, it was
demolished by the sea in that year. The island shown
in fig. 49 as existing in 1649, still further to the east, is
now the Lorelei Bank, and covered by four fathoms of
water.
Eeference to the Admiralty Chart shows that north-east
of the present island are long reef-like banks, large parts
of which are only just covered at low tide. These banks
have a N.W. and S.E. direction parallel to that of the
rocky scars around Heligoland, and their surface is partly
bare rock and partly stones. A bottom of rock and
stones is frequent all over the area encircled by the 10
fathom contour line, an area which measures 5 miles from
north to south, and about 3a from west to east, and is
united by a narrow neck to the shallow ground on the
east, of which the Lorelei Bank is a part. These facts are
suffilcient to show that the old map is to a large extent
' ThLs map, with a note by Mr. W. H. Penning, was published
in the "Geol. Mag.," 1876, Dec. 2, vol. iii. p. 283, and I am
indebted to the editor for the use of the block.
CHAP. X.J
MARINE AGENCIES.
181
confirmed by the features of the sea-floor around the
present island.
Action of Tidal Currents.—The consideration of tidal
currents has been rather neglected by geologists, and it is
very difBcult to form an adequate conception of their
action because the process is hidden from ordinary obser¬
vation. Some facts have been recorded, however, which
seem to indicate that these currents exercise a certain
amount of erosive action on the sea-floor, more particu¬
larly where they pass between islands, or round head¬
lands.
The height of the tide, and the force of the currents it
produces, depend upon the conflguration of the coast-line.
In the open ocean it is merely a slight lifting of the sur¬
face, but the rotation of the earth converts this into a long
low wave, the movement and mass of which become ap¬
preciable as it approaches the land. At the headlands,
which are first reached by the tidal wave, the rise of water
is seldom more than six feet, but where shelving shores
and narrow gulfs or bays confine the tidal waters into a
narrower space, the rise and fall of water often amounts
to 30 or 40 feet. In the Bristol Channel, for instance, it
is more than 40 feet, and the currents produced are pi'O-
portionately powerful.
Where islands occur which present a considerable length
of coast to the advance of the tidal wave, this is divided
into two portions, which travel round the island in oppo¬
site directions, and the configuration of the main coast
beyond the island may cause the one portion of the wave
to rise to a higher level than the other portion. In such
circumstances a strong current or race wiU be developed in
the channel between the island and the mainland. Such
races are common among the islands off the west coast of
Scotland, and where the channels are narrow counter cur¬
rents are often set up with dangerous eddies and whirl¬
pools. The famous Maelstrom or Whirlpool of the Lofoten
Islands of Norway is due to this cause, as are also the
currents of Hellgate in the narrow channel between New
York Bay and Long Island Sound. " If the waters of the
sound could be separated from those of the bay by a
partition at this point, the water at high tide on the
182
dynamical geology.
[sec. ii.
sound side would stand five feet higher, and at low tide
five feet lower, than the water on the bay side." '
It is a mistake to suppose that tidal currents are merely
surface streams, they affect the whole body of water in
the channel or bay, and in some cases the velocity of the
current at the bottom is greater than that at the surface.
In the Little Minch off the west coast of Scotland, be¬
tween Glas Island and Sgeir-i-Noe, the flood-stream often
takes long fishing-lines down to the bottom. " It is a re¬
markable circumstance, indicative of the great depth of
the tidal stream here, that the buoys, though anchored in
70 or 80 fathoms, are taken completely to the bottom,
star-fish and other marine animals being found attached
to them." ^ As in fine weather the surface velocity here is
only li knots per hom*, it is evident the bottom velocity
must be greater. In the Gulf of Coirebhrecain, between
Scarba and Jura, there is a dangerous race where the tide
runs with a velocity of 9^ miles an hour, causing eddies
and counter currents along each side of the gulf, which
has a maximum depth of 80 to 100 fathoms ; and Mr.
Reade rem arks-that the greatest depth occurs just in the
position it should do if it were due to the excavating
power of the current.
In this gulf, and in some other deeps off the Scottish
coast, bare rock is marked on the Admiralty charts, while
all around is mud or sand, and Mr. Reade (loc. cit.) thinks
it possible that most of these valley-like hollows have been
excavated by the action of the tidal currents, for though
their<ferosive influence on a rocky bottom may be small,
on a soft bottom it may be great, and such a current may
have scooped away the sand and clay which filled the
hollow till it reached the rock below. By others these
deeps are regarded as submerged portions of ancient
valleys and lakes, but there is much to be said for Mr.
Reade's view.
Tidal currents certainly seem to have more erosive
power than they are generally credited with, for it is
otherwise difiScult to account for the depth of water out-
' Hinman's " Physical Geography," 1890, p. 130.
' "Sailing Directions for West Coast of Scotland," p. 119,
quoted by M. Reade in " Phil. Mag.," 1888, p. 339.
CHAP. X.J
MARINE AGENCIES.
183
side coasts whicli are known to have been receding at the
rate of several yards a year. Thus Lyell records that in
one part of Sherringham harbour, on the Norfolk coast,
there was in 1829 a depth of 20 feet of water where forty-
eight years before there stood a cliff 50 feet high. Now
if the waves were the only agent of erosion, the tract over
which they had advanced would have been a shallow flat,
and would not have been eroded to such a depth. Mr.
Clement Eeid has recognized this consideration, and adds
a remarkable instance of submarine erosion which came
under his personal notice while surveying the Norfolk
coast. This was the deepening and scouring out of a sub¬
marine channel during a storm to a depth of 15 fathoms,
and the tearing off of slabs of ironstone from the rocky
side of this channel at a depth of at least 10 fathoms,
these slabs being cast up on the beach.'
Bottom currents appear to exist even in the open ocean
and at considerable depths. Thus Mr. Stallibrass records
the existence of strong currents at a depth of 1,000 fathoms
between the Canary Islands, and says their scouring
action may be clearly detected. Mr. J. Y. Buchanan also
remarks : " If we find hard ground, we know that there
must be something to prevent the accumulation of sedi¬
ment. Now the only thing that prevents this is a cur¬
rent ; and one help that telegraph soundings have given
to geographical science is, the indication that tidal cur¬
rents exist even at very great depths in the open sea." "
It has lately been pointed out that in early geological
times the rise and fall of the tides was in all probability
much greater than it is at present. The moon has been
gradually increasing its distance from the earth, and when
it was much nearer, its tide-producing power will have
been greater ; and greater tides would enable the waves to
act over a greater vertical space. Hence Mr. G. H. Darwin
and Dr. Ball ® conclude that marine erosion must formerly
have proceeded at a much more rapid rate than at present,
' " Geology of the Country round Cromer," Mem. Geol. Survey,
1882, pp. 43, 130.
^ " Joum. Soc. Tel. Engineers," 1887, p. 509.
' G. H. Darwin, "PhU. Trans.," 1879, and Dr. Ball in " Nature,"
vol. xxviL p. 201.
184
DYNAMICAL GEOLOGY.
[sec. ii.
but it is by no means certain that such a conclusion is
warranted by the facts. We have seen that marine ero¬
sion is only very rapid in certain cases where the waves at
high tide beat against the foot of a line of cliffs, and that
even in such cases the rate of waste depends greatly on
other circumstances, such as the hardness of the rocks, the
existence of springs, and the work of rain and frost.
Wherever there is a beach below the cliffs, the waves are
engaged in hammering this beach and grinding its peb¬
bles into sand during the greater portion of the time be¬
tween high tides.
Consequently a greater rise and fall of the tide would
not do much toward accelerating the waste of the land,
though it would doubtless create stronger currents, and so
somewhat increase the amount of submarine erosion in
certain localities.
If great tides mean great erosion, then we ought to find
some evidence of such a result at those places where great
tides now exist. The Bay of Fundy (where the tide rises
60 feet) and the Bristol Channel (where it rises 40 feet)
are such places, but their shores do not show any signs of
excessively violent or rapid erosion. Cliffs exist in both
cases at about high-water mark, and below them lies a long
stretch of gradually sloping beach.
The recession of these cliffs is no doubt great in some
places, but no one has yet suggested that it is more rapid
than at other places where rocks of a similar kind are
exposed to marine erosion.
Coast-ice.—In high latitudes where the shallow water
along the coasts is frozen every winter, a particular form of
ice comes into play, and constitutes an erosive and detritive
agency of no mean importance. During the winter a thick
and often broad shelf of ice is gradually formed along the
shore, its thickness increasing with every tide, until it
sometimes reaches a height of 30 or 40 feet, and its sea¬
ward face presents the appearance of a bold wall of ice.
When spring comes, landslips occur, and tons of stones
and rock-fragments, loosened by the winter's frost, fall
upon the surface of the ice-foot, as this shelf is often called.
Moreover, the blocks, stones, and shingle which rest on
the shore beneath are frozen into the bottom of the ice
CHAP. X.J
MARINE AGENCIES,
185
and become part of its mass. The coast-ice is thus charged
with a load of stones and boulders both on its upper and
lower surfaces, just in the same way as glacier-ice is, only
the quantity of debris is probably greater.
As the thaw proceeds in summer time, this ice-foot is
broken up by the waves, and large masses are driven on to
the shore during storms, crushing and grinding the rocks
over which they are pushed ; other portions are separated
from the coast and floated out to sea, forming great rafts
ox floes, each with its load of rock debris.
Even in calmer weather it is clear that coast-ice, having
numerous fragments of rock fixed in it, and being borne
backwards and forwards by the tide, must exercise an
enormous grinding and grooving action. Both the stones
themselves and the surfaces over which they are moved
will be scratched and abraded, and much mud will be pro¬
duced by the process. It may be observed, also, that a
stone fixed in the ice-foot, and scratched along one side by
contact with the rock below, may be dropped when the ice
begins to thaw, and afterwards ground along its other
sides by the stones still fixed in the ice above, so that it
may eventually bear scratches all over it, as is the case
with so many stones in our Boulder clays.
Speaking of ice-action on the coasts of G-rinnell Land,
part of North Greenland, Messrs. Fielden and Be Eance
make the following observations : '—" Sea-ice driven on
shore by gales, or moving up and down with the tides, is a
very potent factor in glaciating rocks and pebbles. Along
the shores of the Polar basin this process of glaciating was
seen in progress by one of us ; and he records in his
' Journal ' that at the south end of a small island in
Blackcliff Bay (lat. 82° 30 N.), the bottom of the ice hum¬
mocks, some 8 to 15 feet thick, were studded with hard
limestone pebbles, which were rounded and scratched as
distinctly as others taken from moraines ; when extracted
from the ice, only the exposed surfaces, as a rule, were
glaciated. As the tide recedes the hummocks do not
always arrive at a position of rest without some distur¬
bances of the subjacent material, particularly on a shelving
^ "Quart. Journ. Geol. Soc.," vol. xxxiv. p. 566.
186
dynamical geology.
[sec. ii.
shore, and the sliding of the hummock to a lower level, and
the sound following on the grating together of the pebbles
beneath may be noted. In many places where gaps
occurred in the lines of ancient sea-terraces, the basement
rock, as weU as some of the pebbles in the terraces, were
found to be glaciated, and there can be no doubt that this
is due to the action of shore-ice, the condition of the ter¬
races precluding the idea that it might have been the
result of glacier action."
An illustration of the action of such ice on a shelving
shore in a much lower latitude is mentioned by Dr. Forch-
hammer,' who states that during a hard frost in February,
1844, sheets of ice were formed in the Sound between
Sweden and Zealand, and some of these were driven by a
storm into the Bay of Täarbejik, and forced up on the
beach, forming a mound more than 16 feet high. When
he visited the spot next day, he saw " ridges of ice, sand,
and pebbles, not only on the shore, but extending far out
into the bottom of the sea, showing how greatly its bed
had been changed, and how easily, where it is composed of
rock, it may be furrowed and streaked by stones firmly
fixed in the moving ice."
To the action of such masses, but still more to that of
ground-ice which forms on shallow bottoms, and contains
stones frozen into its under surface. Professor Forch-
hammer attributes the striation of the rocky surfaces
round the shores of the Baltic.
When to the ordinary action of tides and winds is added
the impact of some of the large masses of ice, which are set
loose from the Arctic regions in summer time, and are
often driven on to the more southern coasts, still greater
results are produced. The action of this pack-ice on a
shelving shore is thus described by Professor Milne : '—
" When we reflect upon the immense mass in one of these
moving fields of ice, we can hardly conceive the energy
that is stored within it. Everything has to give way before
it ; and the coast-ice, with its set of gravers firmly bedded
in its base, is pushed high and dry, sometimes as much as
' " Bull. Soc. Géol. de France, 1847," tom iv. p. 1182.
' "Geological Magazine," Dec. 2, vol. iii. p. 405.
CHAP. X.J
MARINE AGENCIES.
187
100 yards back from high-water mark. It is in this way,
by the coming-in of the northern pack, the rise and
fall of the tide, and other causes, that the land-ice is
driven ashore, and many of the scratches and grooves so
common round the coast of Newfoundland have been
made."
Sir G-eorge Nares observed that in some places on the
coast of Grinnell Land huge floe-bergs and ice-blocks had
been forced up on the inclined shore, until a rampart-like
barrier of solid ice-blocks was accumulated, measuring
about 200 yards in breadth, and rising 50 feet high.'
The impact of this floe-ice seems also to have a rounding
and moulding eflfect upon such islands and prominences of
rock as are exposed to its force, producing forms which, on
a large scale, resemble the roches-moutonnées formed by
glaciers. Professor Milne instances a small islet called
Funk Island, near Newfoundland, which is situated directly
in the course of the ice-floes coming southward from BafBn's
Bay and Labrador. " The northern end of this island,
which has every year to face the pressure of the vast fields
of ice which are l3ome down upon it, is visibly worn down,
and covered with erratic boulders, whilst the opposite
extremity is a low but abrupt cliff."
He argues, therefore, that much of the stratching and
modelling which has hitherto been referred to the action
of enormous glaciers, may have been produced by the
agency of coast-ice and floe-ice acting on the surface of the
country during its last slow emergence from the sea.
The part taken by these ice-floes in transporting and
depositing material will be described on a future page.
^ " Voyage to the Polar Sea," vol. i. p. 276.
SECTION II. [continued).
B. Processes of Beconstruction.
CHAPTEE XI.
T the commencement of Section II. it was stated that
in considering the agencies concerned in the continual
detrition and renewal of the earth's surface, we should first
describe their destructive and dispersive effects, reserving
all mention of their reconstructive effects to a future
section. Accordingly we now proceed to give some account
of the manner in which new deposits are formed out of the
materials obtained by the Several agencies already de¬
scribed, and to indicate the nature of the formations thus
produced. These may be divided into four classes, accord¬
ing to the conditions under which they have been accumu¬
lated, each except the second being subdivided into three
groups according as they owe their formation to mechanical
agencies, to chemical action, or the growth of organic
matter. The deposits formed by rivers and glaciers, how¬
ever, are all the result of mechanical action.
This classification is set out in the following table :—
Terrestrial
Eain, frost, and wind . Mechanical.
Springs and percolating
Worms and plants
Eivers . . . ,
water
Chemical.
Organic.
Lacustrine
Fluviatile
G-laciers
Evaporation of water
Subsiding sediment
I Mechanical.
Mechanical.
Chemical.
Animal agency. .
Vegetable agency .
I Organic.
CHAP. XI.J
TERRESTRIAL DEPOSITS.
189
Subsiding sediment . • 1 Mechanical.
^ . I Action 01 sea-ice ... J
arine Evaporation of water . Chemical.
Animal agency ... Organic.
TEREESTEIAL DEPOSITS.
§ 1.—Mechanically formed.
1. Soil and Rain-wash.—It was shown in Chapter VI.
that the heat of the sun, the fall of rain, and the freezing
of water exercise a powerful effect upon the surface of all
rocks at or near the surface of the earth, and lead to their
gradual decay and disintegration.
In temperate and humid climates rain is by far the
most potent of these agencies; acting in the first place
chemically by means of the acids and oxygen which it
contains, it decomposes and dissolves some of the consti¬
tuents of the rocks exposed to its influence, and so loosens
the coherence of their constituent particles that they are
rendered soft and crumbling to a greater or less depth
from the surface. This rotten and decomposed portion of
the rock is then subjected to still further disintegration by
the mechanical action of the falling drops of rain, and the
lighter particles are eventually washed away by the rills
into which the rain-drops collect, while the residue remains
to form the soil of the district.
The various kinds of soil which mantle the earth's sur¬
face, and conceal the underlying rock or subsoil, have been
primarily produced in this way, the formative action being
assisted, as we shall presently see, by the agency of plants
and earth-worms. The nature of the soil, therefore,
depends largely upon the character of the rock beneath, and
is heavy or light according as argillaceous or arenaceous
matter preponderates in the rock from which it has been
formed. Its jquantity, too, depends partly on the kind of
rock acted upon, its capacity or incapacity of resisting dis¬
integrating agents, and partly upon the contour and slope
of the groimd.
It has been ascertained that the soil has everywhere a
190
dynamical geology.
[sec. ii.
tendency to descend from higher to lower levels : wher¬
ever there is a slope the soil is bodily but slowly travelling
from the top to the bottom, and the steeper the slope the
more rapid is the motion ; so that in valleys and bottoms
where it is not carried away by streams, considerable de¬
posits of rain-wash are gradually accumulated. If a trench
is cut dowu the side of one of our English valleys, it will
be found that on the upper slopes the soü is thin, and the
subsoil may perhaps be within six inches of the surface,
while lower down it will be concealed by 5 or 6 feet of
accumulated rain-wash. In some valleys such rain-wash
is thick enough to be dug for brick-earth. In other places
rain-wash consists of loamy sand or even fine gravel.
Dr. Darwin made some interesting experiments with
the view of estimating the average rate at which the soil
moves,' and he found that on a grass-covered slope with an
inclination of 9i degrees, 2-4 cubic inches of earth (weigh¬
ing I'85 oz. whilst damp), annually crossed a line measuring
one yard in length. " This amount," as he says, " is small,
but we should bear in mind how many branching valleys
intersect most countries, the whole length of which must
be very great, and that earth is steadily travelling down
both turf-covered sides of each valley. For every 100
yards in length in a valley with sides sloping as in the
foregoing cases, 480 cubic inches of damp earth, weighing
above 23 lbs., wiU annually reach the bottom. Here a
thick bed of alluvium will accumulate, ready to be washed
away in the course of centuries, as the stream in the middle
meanders from side to side."
On steeper slopes the motion of the soil-cap is much more
rapid, and in humid climates it sometimes becomes very
obvious. Thus Dr. Coppinger observes:—"The most
characteristic feature in the scenery of the western shores
of Patagonia is owing to the phenomenon of ' soil motion,'
an occurrence which is here in a great measure due to the
exceptionally wet nature of the climate. This slippage of
the soil-cap seems in this region to be continually taking
place wherever the basement rock presents a moderately
inclined surface. . . . The soil-cap takes with it in its
1 " Vegetable Mould and Earth Worms,'' 1881, p. 269.
CHAP. XI.J
TERRESTRIAL DEPOSITS.
191
downward progress not only its clothing of trees, ferns,
and mosses, but also a ' moraine profonde ' of rock-stones
and stems of dead trees, great and small, whereby the
kills are being denuded, and the valleys, lakes, and channels
gradually filled up." ^
The soil is a thick, mossy, peaty, and spongy mass, and
under the influence of gravitation, aided by alternate ex¬
pansion and contraction, this slides down and drags with it
the stones and boulders in the subsoil, till it reaches the
border of a stream or the sea-shore, when its finer portions
are carried away and the larger contents - are left to
accumulate.
In the Falkland Islands are some curious phenomena
known as "done-rivers" ; these are deep accumulations of
large blocks of stone which occupy the bottoms of the
valleys, and though the gradients are slight, they move
slowly and gradually down the valleys till they terminate
in the sea after the manner of a glacier. The blocks are
derived from certain bands of a hard rock, called quartzite,
which are exposed along the sides of the hills and ridges ;
every little glen contributes its stream of angular frag¬
ments to swell the volume of that in the main valley. The
movement appears to be due chiefly to the motion of the
•spongy soil-cap on which they rest, aided by the action of
the trickling rain-water, which flows among and below the
stream of blocks, and by the effects of alternate expansion
and contraction of the blocks themselves.
Mountain Screes and Breccias.—In hilly and
Tocky districts, the materials accumulated by the action of
rain and frost are much coarser than ordinary soils, and
form deposits of angular gravel, which, in limestone regions,
are sometimes cemented by the infiltration of carbonate of
lime, and converted into what is called breccia.
In mountain districts every steep slope or cliff has its
talus of fallen fragments produced by the agencies men¬
tioned in Chap. VI. They are frequently of great length and
breadth, and good examples may be seen in the hill dis¬
tricts of England and Scotland, where they are called screes.
' "Cruise of the Alert,"' p. 77, and " Quart. Journ. Geol. See.,"
vol. xxxvii. p. 348.
192
DYNAMICAL GEOLOGY.
[sec. il
The enormous accumulations of debris among the Eocky
Mountains, forming talus slopes 4,000 feet long, and 1,000
feet thick, have already been mentioned (p. 104).
The materials of a talus or débris slope are always
travelling doirôward, being moved not only by gravitation,
rain and snow, but by the mere alternations of tempera¬
ture which on an exposed hill-side are often very great.
When a stone resting on a slope is heated by the sun, its
greatest expansion is in the direction of least resistance,
that is along the downward side or edge ; again, when
cooled at night, its own weight prevents it from contract¬
ing so much upwards from the bottom as downwards from
the top ; both forces, therefore, tends to cause a slight
downward movement of the stone.
In North Carolina, U.S.A., there are extensive spreads
of angular débris, which have been described by Mr. W.
C. Kerr. Some of the rock-fragments of which they are
composed have travelled a distance of 6 miles from their
parent sites, and now occur at the bases, or on the lower
slopes of the mountains whence they have been derived.
" They have crept down the declivities of the hills, exactly
as a glacier descends an Alpine valley, by successive freezing
and thawing of the whole water-saturated mass, both the
expansion of freezing and gravitation contributing to the
downward movement." ^ Snow also falling on such talus-
slopes, and melting in the summer, would assist in giving
motion to the mass.
Sir A. Eamsay acounts for the formation of the lime¬
stone breccias of Gibraltar in the same way.'' These consist
of " an accumulation of angular blocks of limestone em¬
bedded in a matrix of calcareous grit and earth, the whole
forming a rock-mass as solid as the Gibraltar limestone
itself. It varies in thickness from a few feet up to 30 or
40 yards. This is an old deposit, but he also describes a
more recent one which has a similar composition, and
occupies the position shown in fig. 51.
In this and other cases, where a coast-line has been
elevated the angular fragments and débris of all sizes
' " Keport of the Geol. Survey of North Carolina," vol. i. p. 156.
' "Quart. Journ. Geol. Soc.," vol. xxxiv. 1878, p. 505.
CHAP. XI.]
TERRESTRIAL DEPOSITS.
193
whicli have fallen from the cliffs and slopes accumulate
on the ground beneath, and sometimes form deposits
from twenty to thirty feet in thickness. In Devon and
Cornwall these accumulations are known by the name of
" head."
Sir Charles Lyell observed a breccia in the course of
formation at the pass of Escrinet, near Aubenas, in France.^
" Small pieces of disintegrating limestone are transported,
Fig. 51. Deposits below Windmill Hill, Gibraltar.
Reproduced by permission from " Quart. Journ. Geol. Soc.,"
vol. xxxiv. p. 527.
a. Limestone breccia.
s^. Calcareous sandstone, the re-arranged débris of s^.
s^, Calcareous sandstone, a bedded marine deposit.
I, Gibraltar limestone.
p' and p^, Platforms of marine erosion.
during heavy rains, by a streamlet, to the foot of the
declivity, where landshells are very abundant. The shells
and pieces of stone soon become cemented together by
stalagmite into a compact mass, and the talus thus formed
is in one place 50 feet deep, and 500 yards wide. So firmly
is the lowest portion consolidated, that it is quarried for
millstones."
Clay with Flints and Red Earth.—In the case of
^ "Principles of Geology," tenth edition, vol. ii. p. 522.
O
194
DYNAMICAL GEOLOGY.
[sec. ii.
limestones, which are wasted chiefly by chemical action,
the insoluble residue remains on the surface of the parent
rock, wherever the slopes are not steep, and forms the
superficial soil. Thus, some chalk districts in the South
of England are covered with a peculiar soil, called the clay
with flints, the formation of which is explained as follows :
—White chalk is nearly pure carbonate of lime, the per¬
centage being from 96 to 98, with only 1 or 2 of insoluble
matter ; but it contains nodules of flint, which are like¬
wise insoluble. The carbonate of lime is slowly, but con¬
tinually, removed in solution by rain-water, and the insoluble
residue gradually accumulates, forming a reddish loamy or
clayey soil, full of flints. It is difficult however to separate
the soil formed in this way from that derived from the
reddish loams and clays which once overlay the English
Chalk. This difficulty has led some geologists to deny the
truth of the view that any clay with flints has been formed
from the chalk, but the undoubted fact that similar red
soils occur on limestone plateaux all over the world is a
proof that they are so formed.
Such red earths are found in the hollows and channels
of the Karren-felder mentioned on p. 88, and over the
limestone districts of Istria and Dalmatia, where it is
known as terra rossa. The same material forms the soil
of the limestone plateaux of the Morea, and deep accu¬
mulations of it occur in places where it has been washed
into hollows and depressions ; in the rainy season all the
streams are heavily charged with the red earth which has
been washed into them, and this is carried down into the
Tcatavothra, described on p. 118. The limestone districts of
the Southern States of America are everywhere covered by
similar red residual clays.
Jamaica also affords a good illustration of the way in
which red soils result from the subaerial detrition and
solution of large tracts of limestone. Thick white lime¬
stones occupy more than half the surface area of this island,
and form plateaux which are from 2,000 to 3,000 feet
above the sea, and on which all the phenomena which such
districts usually exhibit are developed on a conspicuous
scale;—deep red soils, calcareous breccias, deeply eroded
rocks, swallow-holes and subterranean caverns. The higher
CHAP. XI.]
TERRESTRIAL DEPOSITS.
195
raised coral-reefs of Barbados are also covered with a deep
soil of red loamy clay.
Tbe Bermuda Islands in the Atlantic afford another in¬
structive instance. Except vrhere covered by blown sand
the surface rock of these islands is recent coral limestone,
and it is everywhere covered by a red residual clay formed
from the solution of the limestone. Sir Wyville Thomson
has given analyses of this clay and of the coral rock from
which it has been derived, and these show how relatively
small are the quantities of the clay-forming materials in
the original rock.'
Constituents.
Red Soil.
Coral Rock.
Combined water and moisture .
18-265
0-328
Silica (as sand and clay) . . .
45-156
0-052
Alumina
Peroxide of iron
15-473
18-898
j- 0-540
Lime
3-948
54-496
Carbonic acid
2-553
44-521
Sulphuric acid
Trace
0-214
Magnesia
0-539
1-751
Potash and soda
0-140
0-318
Phosphoric acid
0-704
0-080
100-656
102-300
Mr. J. C. Bussell has collected and discussed the analyses
of some of the American red soils, and thinks they may
be regarded as impure ferruginous kaolins.^ They differ
from ordinary clays which have been formed by mechanical
disintegration and transport in the following particulars :
they contain larger proportions of iron and combined water,
they consist mainly of fine amorphous matter, compara¬
tively free from the crystalline particles which are so nume¬
rous in ordinary clays and brick earths.
* " Voyage of the Challenger,'' vol. 1. p. 325. But I have quoted
the recalculated analyses given by Mr. J. C. Kussell in "Bull. U.S.
Geol. Survey," No. 52.
= " BuU. U.S. Geol. Survey," No. 52, p. 40.
196
DYNAMICAL GEOLOGY.
[sec. ii.
Liaterite.—This material may be considered here, though
its mode of formation, being rather uncertain, it might be
given under a different heading. Laterite is a highly fer¬
ruginous clay, generally of a brick-red colour. The per¬
oxide of iron is sometimes present in such quantity that it
forms concretionary nodules of pisolitic iron ore containing
30 to 40 per cent, of metallic iron. When freshly dug it
is soft, and may be cut into blocks which harden on expo¬
sure to the air and form a durable building stone.
Laterite is largely developed in India, especially on the
Deccan plateau, -where it is sometimes 150 or 200 feet thick.
Several explanations of its formation have been suggested.
Sir Charles Lyell thought it resulted from the decomposi¬
tion of a deposit of volcanic dust. Others regard it as a
product of the decomposition of highly ferruginous igneous
rocks, such as basalt ; and this probably has often been
its origin, for lateritic soils are especially thick where
basalt forms the underlying rock, and they are confined
to tropical and subtropical countries where there is a
luxuriant vegetation.
Organic acids formed from decomposing vegetable matter
and carried down in rain-water are doubtless active agents
in its production; and their action is increased by the
climatic influences of such regions, and especially by the
alternating periods of heavy rain and excessive heat, which
are so highly calculated to induce a rapid disintegratiou of
any rock exposed to them. For not only is there an ample
supply of percolating water at one time of the year, but
the heat of the dry season draws some of the underground
water up again, and by its gradual evaporation in the sur¬
face soil causes it to deposit the iron it has leached out of
the rock below in the form of the peroxide.
StiU, this -will not account for all latentes, some of which
are considered by Mr. Blanford to be true detrital deposits
of various ages.^ He points out that they often rest on
gneiss and other rocks, and that their seeming passage
do-wn into these rocks may be due to infiltration of iron from
the laterite.
^ See "Manual of the Geology of India," by Meddlicot and
Blanford.
CHAP. XI.J
TERBESTRIAL DEPOSITS.
197
Cave Earth, &c.—When brooks and torrents are en-
gulphed in open fissures and swallow-holes, the earthy
materials which they carry along in mechanical suspension
are deposited underground in the hollows of their sub¬
terranean channels. Thus the torrents of the Morea are
usually charged with reddish mud, sand, and pebbles,
when they enter the katavothra, but are pure and limpid
when they flow out again. Inside the caverns are found
deposits of red mud, containing pebbles and bones of
various animals which had been washed in during the
floods. Similar beds have been found in the caves of
Malta and Gibraltar.
In most caverns, pieces of rock are from time to time
detached from the roof, and the fra,gments are then em¬
bedded in the deposit forming below, which is frequently
hardened by calcareous infiltrations, and converted into
breccia. Such breccias are generally formed on the rocky
floor of caverns, and are often overlaid by deposits of fine
earth and loam, washed in through the cracks and passages
communicating with the surface of the ground. These
cave-earths are often many feet in thickness, and contain
the bones of creatures which have inhabited the cave, or
have been dragged in by beasts of prey.
Kaolin.—The formation of kaolin clay from granite is
an excellent instance of the disintegration of a rock and the
reconstruction of its materials. The mode in which the
felspar of the granite is decomposed, has already been de¬
scribed (p. 91). The silicate of potash is removed in solu¬
tion by rain-water, and the silicate of alumina is carried
away in suspension. The latter, when re-deposited in
hollows and valleys, forms the valuable beds of kaolin or
china-clay, a hydrated silicate of alumina. Such deposits
contain grains of quartz, scales of mica, and other impuri¬
ties, which are separated by washing before the clay is ex¬
ported to the potteries. Large deposits of kaolin occur in
Cornwall and other granitic districts.
Loess.—In many countries the wind plays an im¬
portant part as a distributer of material, and in tbe for¬
mation of certain terrestrial deposits. Dust-storms on a
small scale often occur in the English fenland during dry
springs and summers, and occasionally a field has been
198
DYNAMICAL GEOLOGY.
[sec. ii.
entirely stripped of its surface soil and of the seed sown in
it. Sand storms occur in West Norfolk, and on the
western side of the Wolds in North Lincolnshire.
Where the soil is dried and desiccated by the sun during
the whole or greater part of the year, dust-storms are of
frequent occurrence, and lead to the formation of enor¬
mous accumulations of dust. The loess of central Europe
and of China, the hlack earth of the Russian steppes, and
the deep soils of Manitoba and Nebraska in North
America, are all believed to be œolian or wind-made
formations.
The European loëss is a homogeneous yellowish or light
brown calcareous loam, breaking vertically, without any
tendency to split into horizontal layers, containing only
bones and shells of terrestrial creatures, and penetrated
by thin tubules of carbonate of lime, which look as if they
had originally coated the roots of plants. This deposit
extends up to great elevations, to 4,000, and in the Car¬
pathians even to 5,000 feet, while on lowlands, as in the
valley of the Rhine, it is sometimes more than 200 feet
thick. The hlach earth of Russia has similar characters,
but the surface portion is mixed with vegetable matter,
which gives it a black colour. Underneath it is of the same
brownish hue.
In North China loëss deposits occur on an enormous
scale, being in some places from 1,000 to 1,500 feet thick,
and forming extensive plateaux at levels of 7,000 or 8,000
feet above the sea. Just as in Europe, the deposit exhibits
no stratification, but has a strong tendency to cleave along
vertical planes, so that it readily yields to the erosion of
running water. These plateaux are in consequence trenched
by a system of gulches and valleys, which present special
and peculiar outlines, and form a curious type of scenery.
The material falls in vertical slices which are easily broken
up and removed by the streams, so that the walls of the
valleys are perfectly vertical, and there is often a con¬
siderable space of level ground between, with here and there
an isolated mass or pillar which has escaped destruction.
Richthofen describes some of these walls as 500 feet high,
and as affording habitation to the people of the country,
who excavate chambers in the soft and yet firm material.
Fig. 52. A Valley in the Loess country of North China (after Richthofen).
200
dynamical geology.
[sec. 11.
The ground above recedes in a series of steps with vertical
faces, and tributary gullies enter the main valley and
exhibit similar features for some distance as they ramify
through the country (see fig. 52).
The manner in which these deposits have originated has
been much discussed, but the only satisfactory explana¬
tion is that of Richthofen, who regards them as the result
of dust-storms. Perhaps, however, he has laid too much
stress on the necessity of arid conditions, and too little on
that of damp surfaces ; the material must have come from
dry surfaces and have settled on damp surfaces, where a
certain amount of vegetation existed, and small land shells
were able to live. The conditions which led to the accu¬
mulation of such huge deposits in China and in Europe
have passed away ; but are still prevalent in some parts of
central Asia. Thus in Khotan the sun is often obscured
by clouds of fine dust, and where this falls a coat of yellow
sediment is spread over the ground. The inhabitants say
that it not only deepens the soil, but renders it more fertile,
and they regard it as a manure of the greatest value.^
Blown Sand.—The sands blown off the shore by the
wind drift into ridges and imdulating tracts, called dunes,
which often spread over considerable areas. On many
shores these drifts continually advance inland and cover
up fertile districts with barren sand ; in other places the
dunes form a bar across the mouth of some estuary, and
cause the accumulation of silt inside which may eventually
become habitable land.
As an instance of the former effect, the sand-drifts of
the north coast of Cornwall may be cited ; these have in¬
vaded a considerable area of cultivated land, and now form
hills several hundred feet above the sea-level. This sand
is largely composed of comminuted shells, and some parts
of it are being converted into stone by the infiltration of
carbonate of lime and oxide of iron.
Of the latter effect there are many instances on our
eastern coast. Thus in Norfolk, between Eccles and
Winterten, hills of blown sand have barred up and ex-
' The student will find the origin of the loess discussed in detail
by Baron Richthofen and Mr. Howorth, " Geol. Mag.," Dec. 2,
vol. ix.
CHAP. XI.J
TEKRESTRIAL DEPOSIT?.
201
eluded the tide from many small estuaries. At Winterten
there is an inland cliff about a mile long and some distance
from the sea. Again, from Happisburgh, southwards,
hills of blown sand extend as far as Yarmouth, where they
spread out into extensive dunes.
In France, along the shores of the Bay of Biscay, the
sands used to advance inland at the rate of 60 or 70 feet
per annum, overwhelming houses and farms in their pro¬
gress, but this has been arrested by the planting of pine
forests.
Accumulations of blown sand are frequent on the coasts
of Australia. Mr. Jukes ^ describes an instance on the
eastern coast near Port Bowen, where large sandbanks are
exposed at low water. " These dry rapidly under the hot
sun and the trade-wind, which blows home upon the shore,
then drifts the sand up upon the beach, and piles it into
hills 50 or 60 feet high. Behind these hills is a large
mangrove swamp, which is being gradually buried under
the advancing sand. . . . Large districts, with hills of 200
or 300 feet in height, are found also on the coasts of
Western Australia, stretching sometimes ten miles inland,
formed of loose incoherent sand, once apparently drifted
by the wind, though now brought to rest by the growth of
a wide-spread forest of gum-trees. Parts of these sands,
which consist greatly of grains of shells and corals, are
compacted together into a stone, hard enough to be used
for building, by the action of the rain-water dissolving
some of the carbonate of lime, and re-depositing it on
evaporation." '
Tracts of blown sand, however, are not confined to the
vicinity of sea-shores ; they are frequent in the deserts of
Africa and Arabia, where they are formed from the dis¬
integration of sandstones and arenaceous limestones. Many
of the ancient temples of Egypt have been buried under
drifts of sand, and the whole Egyptian valley would,
doubtless, have been overwhelmed by the desert-sands if
it had not been for the annual flooding of the country by
the Nile. In Southern India, in the district of Tinnevelly,
^ Jukes' " Manual of Geology," second edition, pp. 154, 155.
^ For an interesting instance of travelling sands read Sir Wyville
Thomson's account of those in Bermuda, in his " Atlantic," vol. i.
202
DYNAMICAL GEOLOGY.
[sec. 11.
a sand-drift is described as advancing in a progressive
wave and overwhelming fields and villages in its course.
These sand-hills are being gradually driven from W.N.W.
to E.S.E., that being the direction of the prevailing winds
in the regions where they are situated. A comparison
between surveyor's marks, fixed at various times since the
year 1808, shows that the whole mass of hills is moving
E.S.E. at the rate of about 51 feet a year. Efforts to
arrest the drift, by planting trees, grass, and creepers, have
as yet proved unavailing.
§ 2. Chemically-formed Deposits.
We have seen that all percolating water is able to dis¬
solve certain portions of the rocks through which it makes
its way, and to hold these substances in solution until it is
evaporated.
When water containing carbonate of lime or other salts
in solution is exposed to evaporation, each drop loses both
water and carbonic acid, and becomes gradually saturated
with the mineral matter, till it can no longer hold it in
solution. Consequently, if the evaporation is continued
beyond this saturation point, some of the dissolved carbo¬
nate must be deposited in a solid form on the surface,
from which evaporation takes place. Deposits formed in
this manner may be considered imder three heads : (1)
Surface incrustations ; (2) Cavern deposits ; (3) Deposits
from springs.
1. Surface Incrustations.—The action of capillarity
in soils and rocks has hardly yet been sufBciently con¬
sidered by geologists. Water falling on dry ground is
conducted downward by capillary action until it reaches
that portion of the rock which is saturated with water,
that is, the surface of the subterranean water-level. When
rain is not falling, and the soil is being dried by evapora¬
tion from the surface, a reverse movement is set up, and
water is drawn upward from the reservoir below, ascend¬
ing by capillary action and passing into vapour at the
surface. The vaporization of moisture from the surface of
the ground is always visible on a hot summer's day, and
it is the ascent of water from below that keeps the soil
CHAP. XI. J TERKESTRIAL DEPOSITS.
203
moist in dry weather below the few inches of earth which
are baked dry and hard. The capillary power of soils and
rocks is greater in proportion to their fineness of grain,
and the consequent minuteness of the pores or spaces be¬
tween the component particles. Thus dry clay or chalk
will take up water more rapidly than dry sandstone, and
will part with it by evaporation more slowly.
It is obvious that as the ascending water is evaporated,
the salts which it holds in solution will be deposited in the
soil or on the ground. Thus it is said that in some parts
of Greece, after the rainy season has ended and rapid
evaporation has set in, saline matters rise to the surface
in such quantity that the herbage is gradually killed by
them, and only the very robust plants continue to grow.
Darwin has explained the incrustations of salt which
occur on some of the mud-plains and pampas of La Plata
by this action. After rain the salts disappear, and all the
water-puddles are saline, but in dry weather the plain is
covered with a crust, which is found to consist partly of
sulphate and partly of chloride of soda.'
Mr. Maw has described a curious calcareous deposit in
Marocco, a hard tufaceous crust, which forms a sheet-like
covering over a large area of country. It varies in thick¬
ness from a few inches to two or three feet, and when
broken often exhibits a banded agate scent structure. The
underlying strata are soft limestones. Mr. Maw refers
its formation " to the intense heat of the sun drawing up
water charged with soluble carbonate of lime, and drying
it layer by layer on the surface. The rapid alternations
of heavy rains and scorching heat which take place on
the Marocco plain, are conditions favourable to this
phenomenon." ^
2. Cavern Deposits.—Stalactite and Stalagmite.—
Drops of water hanging from the roof of a limestone
cavern are often coated over with a delicate film of carbo¬
nate of lime, like the finest tissue-paper. This gradually
forms a little tube, which may be seen sometimes to ac¬
quire a length of some inches, still retaining all its fra¬
gility, until water trickling down the outside of it,
' " Geological Observations on South America,'' chap. iii.
® " Quart. Journ. Geol. Soc.," vol. xxviii. p. 89.
204
dynamical geology.
[sec. ii.
strengthens it by the addition of successive external coats.
TJltimately such tubes will form long icicle-like pendants,
which are called Stalactites and grow downward, while
the drops which fall upon the floor result in an upward
growth, called Stalagmite ; and if the two ultimately unite
a solid column of crystalline rock is produced. The name
stalagmite is also applied to the sheets of the same mate¬
rial which are formed upon the sides and floor of the
cavern (see flg. 32, p. 119 ) ; these often alternate with
layers of earth and sand brought down by underground
waters, so that the cavern is half-filled with deposits of
various kinds. The following beds, numbered from below
upwards, were found to exist in Kent's Hole near
Torquay :—
5. Black Mould at surface.
4. G-ranular Stalagmite.
3. Cave earth.
2. Crystalline Stalagmite.
1. Breccia of rock fragments.
Sometimes in small caves the upper layers of stalagmite
are so thick as nearly to reach the roof, and small holes
and fissures are often completely filled with crystalline car¬
bonate of lime.
3. Deposits from Spring-Waters.—Some springs
are so saturated with carbonate of lime that as soon as they
issue into the air they deposit layers of limestone, which, and
when soft and friable, is called Tufa, and when hard is
called Travertine or Tiburstone. A notable instance of a
tufa deposit has been described by Mr. Maw as occurring
in a ravine cut through the limestone hills near Caerwys
in Flintshire.' The tufa extends up the valley in terrace¬
like ranges for three-quarters of a mile, and is in places
more than 20 feet deep. It is for the most part of a soft
marly nature, enclosing harder masses, which appear to
have accumulated round plant remains, and it contains
land and freshwater shells in abundance. A cavern lined
with stalagmite occurs just above one of the tufa beds and
seems to have given issue to the water which deposited
them, but none now rims through it ; and a stream from a
' " Geol. Mag.," vol. iü. p. 254.
CHAP. XI.]
TERRESTRIAL DEPOSITS.
205
source higlier up the valley has cut a channel from 15 to
20 feet deep through the tufa beds.
Striking examples of travertine deposits occur at the
Baths of San Vignone and San FUippo in Tuscany.' At
the former place a large mass of travertine descends the
hill VFhence the spring issues, and extends for the distance
of more than half a mile. One stratum of it is 15 feet
thick, and is so compact that it serves as an excellent
building stone. At San Filippo the water which supplies
the baths falls into a pond where it has been known to de¬
posit a solid mass 30 feet thick in about 20 years. The
mass which descends the hill here is a mile and a quarter
in length, a third of a mile in breadth, and in some places
attains a thickness of 250 feet. This deposit is abruptly
cut off by a small stream which carries much away in so¬
lution to be utilized or deposited elsewhere.
It has been ascertained, however, that travertine is not
always due to mere evaporation of water, and that some
deposits formed in the water of hot springs are mainly
due to the growth of a jelly-like alga, which has the power
of eliminating carbonate of lime from the water, probably
by absorbing the carbonic acid which held it in solution.
Dr. Ferd. Cohn in 1862 ^ showed that this was the case
with the Sprudelstein of Carlsbad (see p. 114) ; he found
that a jelly-like alga spread over the channels in which
the water flowed, and that when squeezed little hard
grains could be felt in its substance. Microscopical exami¬
nation showed these to be grains of carbonate of lime, which,
originating as minute crystals, grew by accretion into
grains and clusters, and eventually amalgamated to form
solid layers.
This has recently been confirmed by Mr. W. H. Weed,
of the Dnited States G-eol. Survey, who found a similar
growth in the water of the Mammoth Hot Springs of the
Yellowstone Park. He describes the alga as growing in
layers, each containing little crystals and stellate accre¬
tions which grow into grains, as stated by Cohn ; the grains
being plainly visible in freshly formed layers of tufa, but
indistinguishable in the older layers, where the grains are
' Lyell's " Principles of Geology," tenth edition, vol. i. p. 400.
' " Abhand. Schles. Gesell. Naturwiss," part ii. p. 35.
206
DYNAMICAL GEOLOGY.
[sec. ii,
cemented together, and the oolitic structure is lost. This
cementation of the grains, and of the laminae containing
them forms a firm more or less compact mass of tra¬
vertine.
Gi/psuni.—Sulphate of lime is also deposited from some
thermal waters. Certain springs in Iceland contain nume¬
rous sulphates in solution which have all been obtained
from the decomposition of the volcanic tuff in the neigh¬
bourhood by the acids dissolved in the water.
Professor Bunsen found that if this tuff was pulverized
and digested for a time in hot carbonated water, nearly all
the constituents were dissolved in the form of bi-carbonates,
the solution containing silicic acid, and the carbonates of
lime, magnesia, soda, and potash, while the insoluble residue
consisted of alumina and oxide of iron. If, however, the
water was saturated with sulphuretted hydrogen, sulphides
were formed, all being dissolved except the sulphide of iron.
In hydrochloric and sulphurous acids all the constituents
were dissolved.
Hence we see that various salts may be formed in such
thermal waters according to the proportion of different
gases with which they are impregnated. The formation
of sulphuric acid, by the oxidation of sulphurous acid in
contact with oxide of iron, would convert the salts into
sulphates, and this probably takes place in the Icelandic
waters. The sulphates in solution are principally those of
lime and magnesia, with smaller quantities of the sul¬
phates of alumina, soda, potash, and ammonia. On eva¬
poration the sulphate of lime is deposited and forms floor¬
like layers of fibrous, sparry material, like the beds of
gypsum so often met with in England. Beds of clay, often
variegated with different colours, are also formed from the
decomposition of the tuff.
Siliceous Sinter.—Other theima springs contain a large
quantity of silica in solution, which is deposited as the
water cools and evaporates, forming layers and incrusta¬
tions of hard siliceous stone called Sinter. Deposits of
this kind occur near the Geysers in Iceland. Analyses of
the water of the Great Geyser show that the amount of
solid matter in solution is about 120 parts in 100,000 of
water, or 84 grains per gallon, nearly half of this being
CHAP. XI. j
TERRESTRIAL DEPOSITS.
207
silica and the remainder consisting of salts of soda and
potash.
Professor Bunsen remarks that the silica is dissolved out
of the felspathic lavas belovr and is held in solution as a
hydrate by the alkaline carbonates and sulphates. He
observes that " no trace of silica is precipitated on the
cooling of the -water, and it is only after the evaporation
of the water that silica is deposited as a thin film on the
moistened sides of the vessel, where evaporation to dry¬
ness takes place, whilst the fluid itself is not rendered
turbid by hydrated silica until the process of concentra¬
tion is far advanced." ^ The result of this is that the in¬
crustations increase in proportion as the surface of evapo¬
ration expands with the spread of the water. Extensive
deposits of siliceous sinter, or Geyserite, as it is sometimes
called, are thus formed, one such being reported as 16
miles long, 2 furlongs wide, and 100 feet thick.
An analysis by Damour showed that the sinter had the
following composition :—
Geysers also occur in New Zealand, and one near the lake
of Eotomahana, formed beautiful terraces of sinter, which
have been described by the Eev. R. Abbaye," but they were
entirely destroyed by the great eruption of Tarawera in 1886.
The Geyser from which the water issued was situated on the
border of the lake, and a hundred feet above its level ; on
the intervening slope the descending waters deposited
their silica and formed a series of terrace-basins. These
basins were mostly semicircular in shape, but varied much
in size, some being very small, others extending for a
length of 20 or 30 yards. The sinter itself contained about
97 per cent, of hydrated silica.
' De la Beche's "Geological Observer," p. 374.
' "Quart. Journ. Geol. Soc.," vol. xxxiv. p. 170.
Silica ....
Alumina and iron peroxide
Lime ....
Traces of soda and potash
Water of hydration
87-21
1-25
1-71
0-66
8-90
99-73
208
DYNAMICAL GEOLOGY.
[sec. ii.
The most wonderful exhibition of the phenomena con¬
nected with Geysers is to be found in the Great Geyser
basin of the Pirehole River in the Yellowstone Park,
United States; Dr. P. V. Hayden thus describes the
scene : '—" Ascending the Pirehole, we find the surface on
both sides the river covered with a thick siliceous crust,
and completely riddled with springs of every variety.
Quiet springs, with basins varying from a few inches to a
hundred feet in diameter, are distributed everywhere.
Some high pyramidal cones, with steam issuing from the
summits, indicate the last stages of what were once im¬
portant Geysers." Professor Geikie, in writing of this
region says : " A prodigious mass of sinter has been laid
down, and the form of the ground has been thereby
materially changed. We made some short excursions into
the forest, and as far as we penetrated the same floor of
sinter was everywhere traceable." Another interesting
feature of the region is the quantity of incrusted and sUi-
cified wood found scattered about the springs.
Recent examination of the sinter which is being formed
m the geyser basins by Mr. W. H. Weed has resulted in
the discovery that this deposit (like the travertine) is often
largely due to the growth of plants.^ In the hotter water
these growths are fllamentous algae which become coated
with sihca, but in the pools of cooler water they form
leathery sheets of tough gelatinous material, green, yellow,
brown, or red, with coralloid and vase-shaped masses rising
to the surface, and often flUing up a large part of the
pool. The tops of these growths unite into a flat crust
which roofs over the pool, and often forms the floor of a
new basin. The masses themselves consist of gelatinous
silica in which the fibrous growth of the alga can be de¬
tected, and when the plants die there is a loss of water
and shrinkage, with a gradual change to a cheesy kind of
substance which appears to be hardened by a further pre¬
cipitation of silica, due probably to the action of the de¬
caying organic matter. The sinter thus formed is opaque,
white, and often chalk-like in appearance, while that
formed by evaporation is translucent, hard, and heavy ;
^ " Amer. Joum. Science and Arts," 1872, p. 161.
^ "Ninth Annual Report of U.S. Geol. Survey," p. 650.
CHAP. XI.J TERRESTRIAL DEPOSITS.
209
both kinds occur in some of the Geyser waters, and
analyses show that the sinter formed hj algous growth
contains less water and consequently a larger proportion
of silica than the other.
In the Grand Cañón of the Yellowstone immense de¬
posits of silica, coloured every shade of red, yellow, and
white, are seen to rest upon the irregular surface of an old
lava-stream. The remarkable beauty of this valley is
largely due to the exquisite delicacy and variety of these
colours.
§ 3. Organically-formed Deposits.
Soil or Mould.—The mode in which ordinary soil or
mould is formed, has already been described ; its dark
colour is due to the presence of decayed vegetable matter,
but the quantity of this in ordinary soil or field earth is
very small. In what is sometimes called vegetable mould
the amotmt is greater. In one sample of fertile mould the
amount of organic matter was ascertained to be only I"76
per cent., in some artificially prepared soil it was as much
as 5*5 per cent., and in the famous black soil of Russia
from 5 to even 12 per cent.^
Dr. Darwin has shown that earth worms are largely
concerned in the preparation of ordinary mould. In making
their burrows they swallow the earth and pass it through
their bodies, and he observes that the particles of the
softer rocks suffer some amount of mechanical trituration
in their muscular gizzards, in which small stones serve as
millstones. He gives several instances in which a layer of
earth two-tenths of an inch in thickness per acre has been
annually brought to the surface by worms ; thus, on good
argillaceous pasture-land over the Chalk at Down, the
accumulation during twenty-nine years amounted to fif
inches of mould between the turf and a layer of chalk
known to have been spread over the field in 1842. This
shows a production of mould at the rate of 2 2 inches in
ten years. Anyone who walks over a grass-plot, and
observes the number of worm-casts, each consisting of
from a quarter to half an ounce of fine black earth, must
' Darwin's " Vegetable Mould and Earth Worms," p. 238.
P
210
DYNAMICAL GEOLOGY.
[sec. ii.
perceive that a very considerable disturbance of the soil is
going on. In other countries larger worms make larger
castings.
" The finely levigated castings when brought to the sur¬
face in a moist condition, flow during rainy weather down
any moderate slope, and the smaller particles are washed
far down even a gently-inclined surface. Castings, when
dry, often crumble into small pellets, and these are apt to
roil down any sloping surface. . . . The removal of worm-
castings by the above means leads to results which are far
from insignificant." ^ (See p. 190.)
Eecent observations in Yorubaland, West Africa, by Mr.
A. Millson, show that the work performed by worms in that
country is still more extensive and important. The soil of
the country is very fertile, and Mr. Millson attributes this
fertility entirely to the action of worms. He says : " The
whole surface of the ground among the grass is seen to be
covered by serried ranks of cylindrical worm-casts, varying
from a quarter of an inch to three inches, and existing in
astonishing numbers. For scores of miles they cover the
land, closely packed upright, and burnt by the sun into
rigid rolls of hardened clay, which stand until the rain
breaks them down into a fine powder." On digging down
the soil is found to be drilled in all directions by worm-
holes, and in the moist subsoil between one and two feet
down the worms themselves are found. By collecting the
worm-casts from a measured space, and weighing them,
Mr. Millson found them to average 5 lbs. per square foot,
which gives a total of 62,233 tons of subsoil brought to the
surface over every square mile every year.
In other parts of Africa, as pointed out by Professor
Drummond, the white ants perform a similar service. The
large hillocks, five or six feet high, raised by these little
creatures are well known ; and Professor Drummond ob¬
served that the old hillocks, under the influence of tropical
rains, gradually moulder down, the material being distri¬
buted over the neighbouring ground.
Even in temperate climes the work done by ants is pro¬
bably considerable, for ants and earthworms rarely frequent
' Op. cit., p. 306.
CHAP. XI.J
TERRESTRIAL DEPOSITS.
211
the same ground, and the renewal of soil on chalk downs,
high sandy commons, and heath lands is largely due to
ants. Old pastures may often be seen in England where
ant-hills are so numerous as to be almost confluent with
one another.
Formation of Peat.—In cold and temperate chmates.
Tarions kinds of plants, which flourish in moist situations,
increase and accumulate to such an extent that their de¬
caying remains form large peat bogs or mosses. In Eng¬
land and Europe generally, some species of moss constitute
the greater part of the mass, but elsewhere other plants
contribute largely to the growth of peat. Darwin ob¬
served that in Tiérra del Fuego almost all the plants con¬
tribute to its production, but especially a plant called
Asteliapumila (Brown), and it is a singular fact that no
mosses enter into the composition of the South American
peat. In the English Fen-country the peat is chiefly formed
of a moss called Hypnum fluitans, but in the mountain
bogs of Ireland and Scotland the moss is a species of
Sphagnum. Excavations in a peat bog show that on the
surface grows the green living moss with many other
plants. Two or three inches below that is a brown spongy
mass, consisting of the flbres of dead plants ; this passes
gradually down into a compacted brown mass in which the
vegetable tissue begins to disappear. Lower down it is
still denser and darker, and all obvious traces of fibre and
tissue perhaps are lost ; until, at a depth of sometimes 30
feet, a compact black substance is found which cuts like
cheese, but, except from it dampness, might be called soft
coal.
The trunks and branches of trees are frequently found
embedded in peat-mosses, especially near the bottom,
where the wood is generally converted into a dark brown
or black cheesy substance, soft when first imcovered, but
hardening on exposure to the air. Such is the nature of
the material known as Irish bog-oak. According to Sir J.
Eennie, many of the peat-mosses in Europe occupy the site
of great forests, some of which have been destroyed within
historical times. The fallen timber, by obstructing the
natural drainage and causing the ground to become wet
and marshy, has conduced to the growth of the mosses
212
DYNAMICAL GEOLOGY.
[sec. ii.
which produce peat. Thus, we are told, that the overthrow
of a forest by storm, about the middle of the seventeenth
century, gave rise to a peat-moss near Lochbroom, in Eoss-
shire, and that in less than half a century after the fall of
the trees, the inhabitants dug peat there.'
Other peat-mosses occupy the sites of silted-up lakes and
pools, which have been gradually converted into swamps.
Many such swamps owe their origin to the damming up of
streams by beavers, so that these industrious animals may
really be ranked among geological agents. The authors of
the " North-West Passage by Land," ^ observe that the
former operations of beavers in Canada must have been
upon a very large scale, for " nearly every stream between
the Pembina and the Athabasca, except the large river
Macleod, appears to have been destroyed by the agency of
these animals. The whole of this region is little else than
a succession of pine-swamps separated by narrow ridges of
higher ground, and it is a curious question whether that
enormous tract of country marked swampy on the maps
has not been brought to this condition by the work of the
beavers, who have thus destroyed by their own labour the
streams necessary to their existence." At one place they
found a long chain of marshes formed by the damming up
of a stream which had ceased to exist, the beaver huts had
become grassy mounds on the dry land, and the dam in
front was a green and solid bank.
Many peat-mosses extend over large tracts of land ; one
of the mosses near the river Shannon in Ireland is said to
be 50 miles long with a breadth of 2 or 3 miles. The
Great Dismal Swamp of North America is described by
Lyell as an extensive morass 40 miles long from north to
south, and 25 miles wide, between the towns of Norfolk, in
Virginia, and Weldon, in North Carolina. " The surface
of the bog is carpeted with mosses and densely covered
with ferns and reeds, above which many evergreen shrubs
and trees flourish, especially the White Cedar and the de¬
ciduous Cypress. . . . On the surface he innumerable
trunks of Wge and tall trees blown down by the winds,
while thousands of others are buried at various depths in
' Kennie's "Essays on Peat," p. 65.
^ Lord Milton and Dr. Cheadle, op. cit., pp. 178, 211.
CHâ.P. XI.J
TERRESTRIAL DEPOSITS.
213
the mire below. The soil to the depth of 15 feet is formed
of organic matter, without any admixture of earth." ^
Near river mouths large tracts of land sometimes pass
into the condition of fens or marshes, parts of which sup¬
port a growth of peat-moss. The Fens of Norfolk and
Cambridgeshire, the Lewes Levels, Pevensey Marshes, and
parts of Denmark and Holland are examples. The surface
peat of the Pen-land varies in thickness from 1 to 18 feet ;
when it is thick enough to be dug for fuel, the uppermost
10 or 12 inches are seen to be dark brown in colour and
consist of an amorphous mass of roots, rushes, and moss ;
the lower portions are quite black, and are bedded in
regular layers, the vegetable structure is. more obscure,
but the mass seems to consist almost exclusively of moss.®
In Norfolk and Cambridgeshire alone, an area of more
than 500 miles is occupied by peat. Excavations in this
Fenland show, moreover, that there have been many periods
during which the peat-moss grew and flourished, for sub¬
terranean beds of peat occur at various depths, as many as
five having been found one below the other, separated by
deposits of clay or silt ; just as coal-seams are separated
by beds of clay or sandstone.
Bemains of forest trees are abundant both in the surface
peat as well as in the subterranean deposits ; indeed these
rehcs are so abundant in many places, that it is evident
that extensive forests have existed from time to time, and
in these localities it may have been the destruction of
the forests that gave rise to the formation of peat. The
trees which grew in these forests were chiefly oak, elm,
birch, yew, willow, and sallow ; their roots and stumps are
frequently found still in the position of growth, and the
fallen trunks are often from 40 to 60 feet long. It is
remarkable that these trunks almost always he in one
direction, viz., with their tops towards the N.E., as if
they had been blown down by winds from the S.W.
We can well imagine that when conditions ceased to be
favourable to forest-growth, and the ground became marshy
' " Principles of Geology," vol. ii. p. 506, and " Travels in N.
America," vol. i. p. 143.
' Skertchly, " Geology of the Fenland," Mem. Geol. Survey,
p. 134.
214
dynamical geology.
[sec. ii.
and boggy, the stagnant water and the growth of peat
would cause the decay of the trees. Thus enfeebled, they
would ultimately yield to the force of storms from the
S.W., and falling, " be entombed in the material which,
their destroyer in life, became their preserver in death." ^
Mangrove Swamps.—In tropical countries similar accu¬
mulations of vegetable matter exist in the mangrove
swamps which are formed along low-lying shores and near
the mouths of rivers. The estuary of a tropical river is
generally bordered by mangrove swamps, and the trees
seem to flourish best where they are within the influence
of the tide. At low water the stems are seen to be sup¬
ported by a tangled mass of branch-Like roots, which rise
out of a bed of thick, black, unctuous mud. Small crabs
and other marine creatures crawl over the mud ; oysters
fasten themselves to the mangrove roots, and often form
huge clusters of many hundreds together ; decomposing
vegetation is everywhere, and the atmosphere of the swamp
is close, damp, and malarious. But as the tide rises and
spreads over the swamp the roots are concealed and the
trees then appear to be growing in the water, presenting
a much more picturesque appearance.
The following description of mangroves and their habits
is quoted from Captain Nelson's memoir on the Bahamas
" The Mangrove (Rhizophora) seldom grows more than 15
feet in height ; the strength and durability of its timber
are very great, and from its development of roots and its
amphibious habit, it is an important agent in the conver¬
sion of swamps and littoral tracts into dry land. There are
many species, but the most common in the Bahamas are
the yellow and the white mangroves. The yellow man¬
grove sends out horizontal roots inland, and into the water
it throws down numerous vertical radicles or branch-like
roots (to which á variety of living things soon become
fixed) ; it also throws off bud-plants, which dropping from
the surface into the water, float until they attain a mud-
bank or rock-head or other congenial point of fixture. The
white mangrove throws down no pendants, but on every
quarter it throws off roots which penetrate the mud hori-
' Skertchly, op. cit., p. 164.
" " Quart. Joum. Geol. See.," vol. ix. p. 210.
CHAP. XI.J TERRESTRIAL DEPOSITS.
215
zontally about six or eight inches under the surface, and
send up suckers at every three or four inches of their
course. Thus each species by the multiplication of roots
and stems becomes an effective agent in the retention and
increase of soil, and frequently both combine in their work
of encroachment on the water and the formation of land ;
as is well seen along the north-east shore of Cunningham
Lake. In the swampy clay and sand of the Florida coast,
the mangroves form dense jungles from five to twenty
miles broad, and running up the creeks and inlets."
Captain Nelson also observes that other and much
smaller plants contribute largely to the formation of new
land in the Bahamas. " The marshy lands that are gradu¬
ally taking the place of the creeks and brackish lakes,
abound with and may be said at some points to consist
largely of a highly calciferous moss-like Conferva, which
in concert with mangrove roots, grasses, and other plants
consolidates and completes the chalky soil."
In Brazil also the mangrove is an efficient agent in aid¬
ing the silting up of the large lagoons which border the
coast. When by deposition of sand the bottom is brought
up to the level of low tide the seeds of the mangrove take
root, and the shoal is soon covered with the trees. Among
their roots fine silt is deposited, and the sandbank is over¬
laid with a layer of soft sand, which may increase in thick¬
ness till its surface is only covered at high water. Eeeds,
rushes, arums, and coarse grasses then assist in converting
the swamp into land.'
' Hartt's "Geology of Brazil," p. 222.
CHAPTEE XII.
FLUVIATILE DEPOSITS.
Deposits formed hy Rivers and Glaciers.
Deposition of Detritus.—It must be borne in mind
that the three processes, erosion, transportation, and de¬
position are closely connected ; each may proceed in close
neighbourhood to the other, and the hollow which has been
formed by erosion one day, may be filled up with sediment
on the next. It is seldom that the exact conditions of
equilibrium exist, when the velocity of the stream is suffi¬
cient only for the transport of sediment without exercising
A
Fig. 53.
erosion or permitting deposition. For suppose that at a
given time or place, the velocity is just enough to keep the
load of detritus in suspension, it is clear that a slight rain-
shower causing an increase in the volume of water, would
at once convert the load into an agent of erosion, while
conversely a decrease in the volume would allow some of
the load to subside. Speaking generally, therefore, we
may say that when the load is not employed in erosion,
some of it is being deposited.
In the upper part of a river-course where the slopes are
chap. xii.J
FLUVIATILE DEPOSITS.
217
steep, and tlie current swift, only the larger stones are
allowed to rest ; lower down the valley, where the stream
loses some of its velocity, smaller stones and coarse sand
are deposited, though much of the latter is still carried on¬
ward. Eventually this also subsides, and when a great
river reaches the broad plains which are little above the
level of the sea, it carries nothing but the finest silt and
mud, part of which is deposited, and part is poured into
the sea.
A winding river is constantly changing the position of
its curves, for the stream is continually eating away the
bank on one side of a curve and allowing deposition to take
place on the opposite shore. The cause of this is found in
the fact that the whole of the water in the stream does not
move with the same velocity, and that the line of quickest
motion does not keep exactly midway between the banks,
but follows the course indicated in fig. 53, the arrows
showing the impact of the current at the points a a.
Since, therefore, the velocity of the stream is least off the
points b b, and since decreased velocity means decreased
power to transport detritus, it follows that deposition will
take place opposite b b, and the angles will gradually be
filled up with gravel, sand, or silt, as the case may be.
This is shown by the black parts, c c, in fig. 53.
Gravel Beds and Alluvial Levels.—When a stream
has reached a tract where the lessened slope allows gravel
and sand to be deposited, small areas of these deposits
are usually found to occur at intervals, first on one side
of the river, and then on the other. They are slightly
above the flood level of the stream, because, since they
were formed, it has deepened its channel and consequently
flows on a lower plane than it previously did.
Still lower down, the watercourse itself is generally
bordered by strips of level ground, which form a narrow
tract or plain, with the stream winding from side to side
within it, and over which the waters spread in time of
flood. The soil of this flood plain or alluvial level consists
partly of earth conveyed by rain down the sides of the
valley, and partly of the silt and mud deposited by the
overflow of the stream ; when the stream is in flood, and
overflows this flat and shallow ground, the turbid current
Fig. 54. View of a Kiver-Valley with a Gravel Terrace and more recent Alluvium.
chap. xii.]
fluviatile deposits.
219
is checked, and being nnable to carry all its load of sedi¬
ment, some of it is precipitated to the bottom and remains
there on the recession of the waters ; this muddy soil is
called alluvium.
In descending a valley it will always be found that the
width of the alluvium increases in proportion as the volume
of the stream increases, and within this constantly widen¬
ing plain, the river pursues a tortuous and meandering
course. At the same time the tracts of gravel increase in
size and in height above the river-level, and often form
extensive banks and terraces on either side of the alluvium.
Fig. 54 shows a river emerging from hilly ground and
flowing towards the spectator : alluvial levels are seen on
each side of the valley : on the right (at t) is a tributary
stream winding through the deltoid flat it has helped to
Fig. 55. Section through terraces of River Gravel.
form, and on the left (at g) is a bank consisting of gravel
deposited by the river when it flowed at a higher level, and
before it had cut out its present channel.
In many valleys patches of gravel may be found at various
levels above the river, and sometimes at considerable dis¬
tances from the present stream. These are portions of
still older gravel beds deposited at a time when the river
ran at that level, and before it had cut its channel down to
its present depth. These higher banks of gravel have ne¬
cessarily suffered much from erosion and detrition, and are
generally reduced to the state of disconnected patches ; but
in some cases they are better preserved and remain as more
or less continuous terraces, so that the valley side presents
a succession of such gravel terraces, and a section across it
has the appearance exhibited in flg. 55.
It must not be supposed from this diagram that the
220
DYNAMICAL GEOLOGY.
[sec. ii.
terrace, a, originally extended all across the width of the
present valley from a to d. At the time of its formation
the valley was both narrower and shallower, its limits lying
between a and b, and its western side being indicated by
the broken line. As the stream, continually impinging on
one bank, cut this side farther and farther back, the valley
was both deepened and widened, and the terrace, h, re¬
mains to testify its extension from a to c. Each bank of
gravel was, of course, deposited on the bottom of the valley
for the time being, just as the latest gravel, c, occupies the
bottom of the present valley, and has been deposited since
the portion of ground between c and d has been cut away
by the erosion of the stream.
Fig. 56 is a section through a real terrace of gravel at
Barnwell, near Cambridge, and is drawn to a definite
River Gravel- Gravel-
Cam. pit. Priory. pit.
scale. The extent to which the river Cam has lowered its
bed since the formation of this old gravel flat is shown by
the position of the modem alluvium. At the Priory pits
the gravel is about 20 feet deep, and it contains beds of
fine sand and loam, with many land and river-shells. On
the opposite side of the river is another flat of sand and
gravel at an intermediate level; the Barnwell terrace
being about 25 feet, and the Chesterton terrace 15 feet
above the river.
The diagram, fig. 55, represents the usual relation of
the gravel terraces to the rocky slope below, each terrace
being a separate deposit, and quite disconnected from that
above or below. It frequently happens, indeed, that strips
of the underlying rock are exposed along the slopes which
intervene between the terraces. There are some cases.
CHAP. XII.J FLUVIATILE DEPOSITS.
221
however, where a section through the valley would present
the appearance of fig. 57. Such cases are found where
the whole country has been depressed since the formation
of the great valleys, so that these have been filled up with
deposits of sand and clay during the submergence. Sub¬
sequent elevation of the land has allowed the rivers to cut
new channels through these deposits, and the velocity of
the streams increasing pari passu with the amount of up¬
heaval, their capacity of vertical erosion increased while
that of lateral erosion diminished, so that the width of the
flood-plain has gradually contracted, leaving a series of
benches or terrace levels carved out of the mass of material
accumulated in the valley.
Instances of such terraced valleys have been described
in North America, and are so prevalent that the period of
their formation is known as the Terrace epoch. They also
Fig. 57. Terraces cut out of a silted-up Valley.
occur in Patagonia under similar conditions, as described
by Darwin.^ In such cases the number and levels of the
terraces correspond on each side the stream, but in Eng¬
land there are few instances of such valley-structure, and
terraces seldom occur at exactly the same level on opposite
sides of a valley, except where an alteration in the course
of the stream has caused it to cut through an old terrace,
and so to leave a portion on either side of the present
channel. Where two opposite terraces appear to corre¬
spond, it will generally be found that the level of one is
really lower than the level of the other, and that they have
been formed at different epochs in the excavation of the
valley.
In the north-west of England and in Scotland some
valleys appear to exhibit a structure like fig. 57. The
J "Journal of Eesearches, Voyage of the 'Beagle,'" eh. ix. p. 128,
Minerva Library edition.
222
dynamical geology.
[sec. ii.
valleys of the Mersey and Irwell in Lancashire are said to
be instances of this, and Mr. De Ranee has drawn a sec¬
tion showing the terraces on the western side of the Irwell,
near Pendleton, which are apparently cut out of a con¬
tinuous deposit of gravel. The surface of the river here is
about 70 feet above the sea, and the highest terrace is 100
feet above the river (" Proc. Geol. Assoc.," vol. iv. p. 231).
Here also there has been elevation since the filling up of
the original valley.
Where two streams join, or a tributary enters a main
valley, there is almost always a tract of gravelly deposit in
the angle between the two channels. As the streams
deepen their channels, the point of junction is shifted
further and further down the main valley, and the tri¬
angular gravel-covered area is consequently elongated and
increased. Often also it presents two or more terraces,
and, as Mr. H. Miller has pointed out, these junction ter¬
races are produced independently within the recess, and
may not be on the same levels as the terraces in either of
the valleys.'
That river-terraces are produced thus locally and inde¬
pendently is a proof that their formation is simply a result
of the manner in which rivers erode and deepen their
channels, and not on any variations in the volume of water.
The process of river-terracing may sometimes be witnessed
on a miniature scale where the water from a spring fiows
over a sand fiat. Mr. Miller mentions such a case where
spring water flowed over a smooth beach of fine sand
bordering a river : " It had its curves, its deflection-banks,
at the base of one of which it was stiU scooping with a
force that drove away a brush of sand-grains as if they
had vitality in them ; and opposite these little cliffs it had
its terraced slopes left to one side because the stream had
pushed to the other, and marked by the finest possible
engraving of terrace-lines, Tiot at equal heights where they
chanced to stand opposite." He convinced himself that
all these effects had been produced by a steady volume of
water, unaffected by floods or by falls in the level of the
river below, " but simply by planation at different levels
^ "On River-Terracing," Proc. Roy. Phys. Soc. Edin. 1883, p. 288.
CHAP. XII.J FLUVIATILE DEPOSITS.
223
and to vatying breadths, with a tendency to narrow the
field of its operations as its deepened its course." (Op.
cit., p. 302.)
Alluvial Fans and Terraces.—In mountain dis¬
tricts where small lateral valleys open on to the plain of a
wide valley, the detritus carried down by the torrents
which have excavated the side valleys and ravines is
spread out over the ground in front of them, and forms
large fan-shaped tracts of deposits, sloping outward from
the mouth of the ravine.
I;;,'. 58. Fan at Tigar, Nubra Valley, Ladakh. (Drew.)
Mr. F. Drew has described some excellent instances in
the country drained by the upper part of the Indus, two of
which are by his permission and that of the Geological
Society here reproduced. Pig. 58 shows a fan in the
FTubra Valley ; the radii of this fan are about a mile long,
and slope at an angle of from 6° to 6°, so that its depth at
the mouth of the ravine must be 500 feet.
It is clear that the regular outlines of these fans are due
to the manner in which the issuing stream has coursed at
224
DYNAMICAL GEOLOGY.
[sec. ii.
different times over every part of tliem, depositing detritus
as it flows first along one course and then another, and
gradually building up the fan-shaped cone by constant
additions to its outer surface. Every additional coating
of deposit carries the margin a little further and decreases
the steepness of the slope ; hence small fans have steep
slopes, and large fans have gentle slopes.
When the side valleys are near together the fans often
meet or join one another, and form a continuous slope or
terrace. Such a case is shown in fig. 59, where several
fans have become united into a continuous deposit which
has been trenched by fhe main river, and now forms a kind
of irregular terrace high above the stream.
Formation of Deltas.—It has been stated that in
the lower reaches of a river-course, the stream continues
to widen its valley by lateral erosion, now on one side, now
on the other, though it no longer exerts any erosive action
upon its bottom. On the contrary, it deposits a large
quantity of the sand and silt which it brings down, and
this is spread out over the surface of the broad alluvial
plain in which it flows.
If the river debouch into a lake or into a quiet bay of
the sea, where the coast is not swept by any very strong
currents, this alluvium expands into a still wider area
having a more or less triangular shape, like that of the
G-reek letter A, whence it is called a Delta.
chap. xii.J
fluviatile deposits.
225
The head of the delta is the point where the river origi¬
nally entered the lake or sea ; properly speaking, indeed,
the river ends here just as its valley ends, and its waters
naerely find their way as best they can through the muddy
deposit with which it has choked its own mouth. This
they do by means of several channels branching off from
the main stream, each of which conveys a portion of muddy
water to its own mouth, where the sediment is deposited,
and the extent of the delta is thus continually increased.
The Ehine when it enters Holland is lost in a great del¬
toid flat among a number of bifurcating channels, in which
its waters are mingled with those of the Meuse, Sambre,
and other rivers ; these have all contributed to form the
low marshy ground of which the Netherlands consist.
The delta of the Rhone has many points of interest ; the
amount of sediment brought down by the river has been
mentioned on p. 134; this is discharged into the Mediter¬
ranean, and has formed a delta which has a front of about
50 miles, and its head is 25 miles from its edge. Under
the Roman dominion, 400 b.c., Aries was only 14 miles
from the river mouth, now it is 80 miles, hence this delta
has advanced 16 miles in 2,200 years. The depth of the
deposit is also considerable, a well sunk at Aignes Mortes
passed through 328 feet of marls and clays without reach¬
ing the base of the delta.'
The united deltas of the Po and Adige have a length of
about 70 miles from Ostiglia to the mouth of the Po, and
a seaward breadth of about 50 miles ; but they are conter¬
minous with the deltas of the Brenta, Piave, Taghamento,
and Izonzo, so that a continuous tract of marshy land ex¬
tends for more than 100 miles from the Gulf of Trieste
nearly as far as Ravenna. Adria, which was a seaport in
the time of Augustus, and gave its name to the Adriatic
Sea, is now 20 miles inland, and the whole coast-line above
indicated has increased by a width of from 2 to 20 miles
during the last 2,000 years. All this material has been
brought down from the Alps by the tributaries of the
above-mentioned streams.
Many deep wells have been sunk at Venice, and one of
' C. Martins, cited in Prestwich's " Geology," vol. i. p. 85.
226
dynamical geology.
[sec. ii.
them has been carried to 572 feet without reaching the
bottom of the sands and clays of the delta. The deposits
consist of yellow sands, brown and grey clays, with beds of
lignite and dark carbonaceous sandy clay, sands prevailing
in the lower part, clays and lignites in the upper. Most
of the sands contain marine shells, but freshwater species
occur in the lignites.
The delta of the Mississippi has an area of 12,300 square
miles; it has completely filled up the inlet of the sea
which once penetrated deeply into the North American
coast, and has projected far into the Gulf of Mexico, carry¬
ing out the river by a natural canal 50 or 60 miles beyond
the general edge of its delta. The head of this delta taken
as the point where the river sends off its first bifurcation
(called the Atchafalaya) is about 200 miles distant from
the coast. A well has been sunk at New Orleans to a
depth of 630 feet without reaching the bottom of the
alluvial deposits. This boring passed through alternations
of clays and sands, the latter predominating in the lower
part of the series. There were eighteen distinct beds of
clay, one being 63 feet thick, and their combined thickness
being 266 feet ; several of them are marly, and effervesce
with acid. Four beds of lignite were found, the lowest
occurring at 150 feet. Below a depth of 40 feet the fossils
were all marine and of recent species, showing that the
delta has simply displaced the waters of the Gulf of
Mexico.
The conterminous delta of the Ganges and Brahmapootra
is still larger, for it makes a coast-line of 260 miles in length,
and a swampy flat which is 100 miles wide at a distance of
250 miles in the interior, its whole area being probably
50,000 or 60,000 square miles. An Artesian well was
bored at Calcutta to a depth of 480 feet, the auger pene¬
trating some old land surfaces at several depths, proving
continued depression of the land (Lyell). This well is also
remarkable for the fact that the lower 80 feet of the boring
consisted of sand, shingle, and boulders, showing that the
velocity of the stream and, therefore, the slope of its water¬
course, must have been very much greater at that time,
and that subsequent depression of the land has lessened
the velocity of the stream, and consequently the rate of
CHAP. XII.]
FLUVIATILE DEPOSIT.S.
227
erosion. The Ganges cannot now carry gravel farther
than a point about 400 miles from the sea. There is pro¬
bably round the head of the Bay of Bengal enough river
deposit to cover all England and Wales to a depth of 200
or 300 feet.
So obviously is the delta of the Nile the production of
that river, that Herodotus remarked that " Egypt was the
gift of the Nile," aud that the sea had probably once flowed
up to Memphis, now more than 100 miles from the coast¬
line, the old gulf having been filled up by the Nile mud, as
the Bed Sea would be filled up if the Nile were turned into
it. The edge of the present delta is about 180 miles wide,
but it is now swept by a powerful current, which carries off
most of the detritus delivered into it, so that very little
addition to the delta has been made in the last 2,000 years.
Another result of this current has been to throw sand¬
banks across the mouths of the lagoons known as Lakes
Mareotis, Bourlos, and Menzaleh, shown in the accom¬
panying plan of the Delta of the Nile, fig. 60 ; these are.
228
DYNAMICAL GEOLOGY.
[sec. II.
of course, slowly silting-up, and will eventually be dry
land.
We need hardly observe that river-valleys do not always
terminate in deltas. There are many great rivers which
do not block up their own mouths in this way, but pour
all their detritus directly into the sea. In some cases the
strength and velocity of their currents is sufficient to carry
the greater part of the sediment out beyond their mouths ;
in other cases the estuary into which the river falls is kept
a semicircular mound stretching across the vaUey.
Such a moraine consists of a confused mass of earth,
stones, and boulders heaped together without any regard
to size or weight ; most of the materials have been shot
over the end of the glacier, just as rubbish is thrown down
to form a railway embankment. The greater number of
the stones, therefore, having been carried on the upper
surface of the glacier, wiU not bear any glacial markings
or scratches ; those, however, which were fixed in the under
surface of the ice, and came in contact with the rocks form-
CHAP. Xll.j
FLUVIAÏILE DEPO.SITS.
229
ing the sides and bottom of the valley, will be polished and
scratched on one or more faces.
Old lateral moraines are also found where ancient glaciers
have melted away, these moraines remaining as long hum-
mocky ridges or banks along the sides of the valley, and
consisting of similar materials to those which compose the
terminal moraines.
Examples of such moraines are to be found in many
parts of the British Islands, where glaciers formerly existed,
viz., in North Wales, in the Lake district, in the highlands
of Scotland, and among the hills of Ireland. An excellent
example occurs near Killamey, at the northern entrance to
the Gap of Dunloe, as described by Mr. Du Noy er.^ Three
lunette-shaped mounds of morainic material, sand, gravel
and boulders, are arranged in a rude concentric form, one
beyond the other across the mouth of the gap ; the two
outer mounds are incomplete, their western extremities
having been destroyed, but the portions remaining on the
east side of the stream measure fully one mile in length by
about 100 yards in width, and their south-east ends rest
on the flank of Purple mountain at an elevation of about
400 feet. The inner mound is much more perfect, and is
cut through in its central part by the river Loe ; on the east
side of the river it measures 650 yards in length by 150 in
width, and on the west side it curves round to the S.W. for
a distance of 700 yards. At the north end of the Black
Lake is still another deposit left by the retreating glacier,
as the climate became warmer.
During the later stage of the Glacial period the higher
parts of the Scottish Uplands nourished groups of glaciers
which have left numerous well-marked moraines.'
A large series of them occur in the valley of the Dengh,
a tributary of the Ken (Kirkcudbrightshire). " The mounds
are arranged in concentric lines, sometimes conical in shape,
but generally assuming the form of narrow ridges which
often trend towards the centre of the valley. Eoad cuttings
show that the moraine matter consists of a sandy clay with
numerous subangular stones, which in many instances have
irregular striations." These mounds are traceable down to
^ Explanation of Sheet 173 of " Geol. Survey of Ireland," p. 23.
^ " Mem. Geol. Surv. Scot.," Sh, 9, p. 41 ; Sh. 15, p. 40.
230
dynamical geology.
[sec. ii.
and across the valley of the Ken, where they rest upon a
thick deposit of tough boulder clay.
In the valley of the Holm Burn, another tributary of the
Ken, there is a similar series of moraine mounds, most of
them forming long winding ridges with a marked bend
towards the centre of the valley, and beautifully illustrat¬
ing the common horse-shoe shape of moraines. They con¬
sist of loose gravelly and sandy earth with well-rounded
stones, of which in one section one-fifth were found to be
ice-scratched, but in other places the proportion is much
less.
Till and Boulder Clay.—Ordinary valley-glaciers do
not appear to leave any deposits behind them besides the
moraines above described. The glaciers of Switzerland
have sometimes retreated for considerable distances, leav¬
ing the ground they formerly covered exposed to view ;
consequently, if any deposits were formed underneath the
ice, we should expect it to remain on this ground.
It does not appear, however, that such deposits ever
occur. Professor Bonney,' who has specially examined re¬
treating glaciers for this purpose, thus writes :—" In no
case have I been able to find signs of any deposit resem¬
bling till or boulder clay; the detrital matter, which is
scattered generally sparsely over the slope left bare by the
retreating glacier, has fallen from its surface, like ordinary
terminal moraine," neither could he find any such deposit
under the ice of the glacier itself.
A similar absence of deposits has been remarked in
Norway.^ Professor Spencer noted several points of contact
between glaciers and their rock-beds, but found nothing
like boulder clay under the ice, though he observed a
glacier advancing over an old terminal moraine which
was partially levelled and probably compressed in the
process.^
It is said, however, that these glaciers are too small to
have the power of forming boulder clay, and that in the
present climates of Norway and Switzerland the fine mud
^ "Notes on Glaciers," "Geo!. Mag,,"' Dec. 2, vol. ill. p. 197.
' See " Proc. Geol. Soc. Liverp.," 1872-73, and Spencer in Geol,
Mag. Dec. 3, vol. iv., p. 168.
^ See also Bonney, op. cit., p, 198,
CHAP. XII.J
FLUYIATILE DEPOSITS.
231
is all carried away by the sub-glacial streams instead of
accumulating to form a pad of clay or Ground-moraine, as
it would do if the climate were more severe. It has been
supposed that the case is different in the Arctic and Ant¬
arctic regions, where the glaciers are not confined to the
valleys, but form a confluent sheet of ice spreading over
the whole countiy, and that under these conditions a mass
of debris must accumulate underneath the ice, and may
form a deposit of considerable thickness.
This accumulation must be partly frozen into the bottom
of the ice, but it is supposed that some of it is pushed
along by the moving ice-sheet, and that portions are de¬
tached and allowed to accumulate in hollows and valleys
underneath the ice. Furthermore, it is assumed that the
ice-sheet passes over such deposits, and allows them to re¬
main undisturbed, notwithstanding the erosive powers
usually attributed to moving ice. Finally, it is said that
the great weight of the superincumbent ice will exercise an
immense pressure upon the material below, and compact
the mud and stones into a tough boulder clay.^ This
hypothesis was framed to account for the formation of the
hard and tough kind of boulder clay known in Scotland by
the name of Till, but Professor James G-eikie has since ap-
phed it to explain the origin of all other boulder-clays.
It is not, however, a completely satisfactory explanation,
while as a matter of fact no one has yet observed boulder-
clay in the Arctic regions occupying such a position with
reference to the inland ice as would suggest the inference
that it has been formed by or beneath the ice. There
is, indeed, a general absence of any kind of moraine deposit
over those areas, which are free of ice ; bare rock is almost
everywhere exposed. Professor Nordenskiold, describing
ground which had but lately been abandoned by the ice,
says it exactly resembled the woodless gneiss districts of
Sweden and Finland, the rounded hills of gneiss being
covered with erratic blocks and divided by valleys with
small lakes and scratched rock surfaces, " on the other hand,
no real moraines were discoverable." ^
Again, Lieut. Lockwood describes the ice-cap of central
^ See " Great Ice Age," by J. Geikie, second edition, ch. i.
^ " Expedition to Greenland," "Geol. Mag.," vol. ix. p. 362.
232
dynamical geology.
[sec. ii.
Grinnell Land as free from rock débris except in a valley
confined by high mountains. The ice-wall is from 120 to
200 feet high, and along its foot there were only here and
there small ridges or banks of moraine deposit.' Norden-
skiold observed similar small banks in Greenland.
The following are difficulties in the way of accepting
Professor J. Geikie's theory.
1. No one has yet seen any traces of a ground-moraine
under a retreating glacier or an ice-sheet.
2. Actual observation has only shown that where glaciers
descend a slope they rest on bare rock and push all loose
materials before them ; where they reach more level ground
they pass over any accumulation of débris without much
disturbing it.
3. Boulder clay is not only found in valleys and plains,
but has been spread over high ground and low ground alike
in a thick and continuous sheet.
4. Boulder clay frequently contains fragments of rock,
the parent sites of which are several hundred feet below
their present resting-place. They must, therefore, have
been carried up by the agent which formed the clay, but it
has not been proved that an ice-sheet can move up a slope
for any considerable distance.
5. There is inconsistency in assuming the existence of a
great underlying sheet of ground-moraine, and at the same
time crediting the ice with the power of excavating lake
basins. Neither one nor the other theory has yet been
proved.
It would appear, however, that land-ice can in some way
give rise to the formation of a boulder-clay, that is, a stiff
clay containing many stones. Such clays, mingled with loose
gravelly deposits, are described by Sir Charles Lyell and
others as occurring at several places on the great plain
which lies between the Alps and the Jura. It was stated
on p. 155 that there was a time when the Swiss glaciers
were so large that they extended beyond the Alpine valleys,
and becoming confluent on this plain formed a great sheet
of ice which terminated against the flanks of the Jura. It
is therefore a natural inference that the glacial deposits
Lockwood in Greely's " Three Years of Arctic Service."'
CHAP. XII.]
FLUVIATILE DEPOSITS.
233
now found on the plain were formed during this extension
of the Swiss ice, but it is a curious fact that English
geologists have paid very little attention to them. No one,
so far as the author is aware, has compared these clays
with the British boulder-clays with the view of indicating
points of difference or resemblance ; nor have these Swiss
clays been brought forward as a special proof of the capa¬
city of glaciers to form boulder-clay. Professor Bonney, how¬
ever, informs the writer that he thinks they may fairly be
regarded as proof that land-ice can make a boulder-clay, or
rather that a boulder-clay can result from the extension of
land-ice, but that there is little evidence to show how the
clay has been formed. The general moundiness of the sur¬
face of the deposits suggests, however, that they have been
formed during the retreat and melting of the ice rather
than underneath the advancing mass of ice.
It should be remembered too that in most coimtries
where glaciers now exist they are only remnants of much
larger glaciers, which may have long ago swept the valleys
clear of all loose debris and surface deposits. It may be,
therefore, that the material of glacier-formed boulder-clay
is gathered by advancing glaciers, accumulated during
many centuries, and only finally arranged by the receding
ice. In a future chapter we shall see that boulder clay
can also be formed in a very different way.
CHAPTEE XIII.
LACUSTRINE DEPOSITS.
HE various ways in which lakes have originated will be
discussed in a later part of this volume. For our pre¬
sent purpose they may be classified under two heads,—
(1) Freshwater Lakes; (2) Saline Lakes. Freshwater
lakes always have rivers flowing out of them as well as into
them, and they can therefore be viewed as merely deep and
wide portions of the river-course where the current is
checked and much sediment deposited. Saline lakes never
have excurrent rivers, for they only become sahne when the
quantity of water running into them is less than that
evaporated from their surface.
1. Mechanical Deposits.—Where rivers enter lakes,
deltas similar to those described in the last chapter will be
formed, but these will generally consist of more varied
materials; because rivers flowing into lakes usually run
with greater velocity than when they enter the sea, and are
more liable to floods which sweep down a quantity of coarse
debris ; thus boulders, gravel, and sand, are either continu¬
ally or occasionally poured into lakes, besides the finer
sediment which rivers everywhere carry.
The detritus thus brought down from the surrounding
land is arranged in a definite and peculiar manner, as may
be seen by watching any small stream which enters a pool.
The particles of sand and gravel swept along by the stream
are dropped when they enter the pool or lake, and are
spread out in a fan-shaped deposit, the surface of which is
§ I. Deposits in Freshwater Lahes.
CHAP. XIII.J
LACUSTRINE DEPOSITS.
235
nearly level or gently inclined, ending in a steeper slope
(see fig. 61). The inclination of this slope varies with the
coarseness of the materials. Further supplies of debris
are pushed along the level surface and shot over the edge
of the bank so as to form an additional sloping layer, and
so the process goes on, successive layers being deposited,
sometimes of gravel, sometimes of sand, but all sloping
in the same direction, viz., away from the mouth of the
stream.
All this time, however,'the river is also pouring into the
lake a quantity of finer sediment which" is carried out far
beyond the edge of the delta. The turbid waters of the
river, having a greater specific gravity than the clearer
waters of the lake, sink beneath the latter and often steal
along the bottom for a considerable distance before they
part with their burden of suspended sediment. As De la
Fig. 61. Diagram of a small Delta in a Lake,
m, mud spread on lake floor, d, delta deposit.
Beche remarks, " if a long trough be filled with clean water,
and turbid water be very quietly poured into it at one end,
the mode in which the latter finds its way beneath the
former will be at once seen." The fine mud, therefore,
which is carried onward by the current of the river, will be
evenly spread over the bottom of the lake in advance of
the growing delta.
Besides a large river, there may be other smaller streams
and torrents flowing into the lake, and each of these will
form a small delta and contribute its share of detritus, so
that in course of time the lake may be completely filled up
by the growth of delta deposits.
Pig. 61 may be taken to represent a small delta formed
by a stream coming from the direction a, and resting upon
a stratum of fine mud, m, which has been spread over the
lake floor by the current of the main river. The deposits
236
DYNAMICAL GEOLOGY.
[sec. ir.
being coarse the slope of the delta is steep. Sir Charles
Lyell states that the subaqueous slope of a minor delta in
the Lake of Geneva is four times as steep as that of the
main delta at the head of the lake.
The deposits of the river Khone in Lake Geneva probably
consist of numerous alternations of fine gravel, coarse and
fine sand, silt and mud ; for during the summer when the
snows melt the volume and velocity of the river are
greatest, and large quantities of sand, mud, and vegetable
matter are introduced, but during the rest of the year the
influx is so much less that, according to Saussure, the whole
lake stands six feet lower.'
The river Rhone is thus gradually filling up the lake,
and has already formed a flat alluvial plain sixteen miles
long at its upper end. An ancient town called Port Val¬
íais (Portus Valesiae) stood on the water's edge about 900
years ago, but now it is a mile and a haK inland, so much
new ground having been formed in the interval. It would
be quite possible to calculate the time required for the
river to convert the whole lake into a similar plain, through
which it would then meander to its present outlet at
Geneva.
Sir Charles Lyell tells us that many such filled-up lakes
may be found in the course of the Rhone and its tributaries.
" If we ascend, for example, the valley through which the
Dranse flows, we find that it consists of a succession of
basins, one above the other, in each of which there is
a wide expanse of flat alluvial lands, separated from the
next basin by a rocky gorge, once perhaps the barrier of a
lake."^
In our own country a good instance of such an obliterated
lake occurs at Rossthwaite above Borrowdale, and there
are many in Scotland, where they are known by the name
of lochlands.
In the case of large lakes which are not traversed by any
large river, the deposits are rather different. Lake Superior
is the largest expanse of fresh water in the world, and
numerous rivers run into it, but none form any considerable
delta ; round its shores there are beaches of sand and
^ Ly^elis " Principles of Geology," tenth edition, vol. i. p. 418.
' Ibid., vol. i. p. 419.
CHAP. XIII.J
LACUSTRINE DEPO.SITS.
237
sliingle, and its bottom consists generally of a very adhe¬
sive clay containing shells of the species now living in the
lake. " When exposed to the air, this clay immediately
becomes so indurated as to require a smart blow of the
hammer to break it. It effervesces slightly with diluted
nitric acid, and is of different colours in different parts of
the lake ; in one district blue, in another red, and in a
third white, hardening into a substance resembling pipe¬
clay." '
2. Deposits of Organic Debris. — Carbonaceous.—
Inter stratified with the mechanical accumulations just
described there will generally be some which are chiefly
formed of organic remains. At times and seasons when
much vegetable matter is carried into the lake, this is
spread out and embodied with the mud or sand, giving rise
to a black carbonaceous or lignitiferous deposit.
The quantity of driftwood carried by rivers which traverse
a forest-covered country is sometimes very great, and when
this is arrested by a lake large deposits of lignite may be
formed. The Mackenzie River in North Canada in its
periodical floods sweeps through the pine forests which
border its banks and carries away thousands of trees every
year. Many of these sink in the eddies of the river and
form shoals, which gradually grow into islands. A thicket
of small willows covers the new-formed island as soon as it
appears above water, and their long fibrous roots serve to
bind the whole firmly together. When such islands are
subsequently cut into by the river, they show alternating
beds of lignite, clay, and sand, the tree trunks having de¬
cayed into the blackish brown substance known as lignite,
or wpod-coal.
Dr. Richardson, from whose account the above is con-
densed,® says : " It was in the river only that we could
observe sections of these deposits, but the same operation
goes on on a much more magnificent scale in the lakes. A
shoal of many miles in extent is formed on the south side
of Athabasca Lake by the drift-timber and vegetable
debris brought down by the Elk River, and the Slave Lake
' Lyell, op. cit., vol. ii. p. 422.
^ " Geognostical Observations on Capt. Franklin's Polar
Expedition."
238
dynamical geology.
[sec. ii.
itself must in process of time be filled up by the matters
daily conveyed into it by the Slave River. Vast quantities
of drift-timber are buried under the sand at the mouth of
the river, and enormous piles of it are accumulated on the
shores of every part of the lake."
Calcareous.—In lakes which are fed by springs or by
small streams that do not carry much muddy sediment,
other deposits of organic origin are formed. In such lakes
freshwater shells abound, and their dead shells often accu¬
mulate in such quantity as to form thick layers of what
is called shell-marl. Similar beds are sometimes formed
by the bivalve shells or carapaces of the small crustácea
called Cyprides, which often swarm in such pools and
lakes. Lastly, the pond-weed called Chara secretes so
much carbonate of lime in its stalks, leaves, and fruit,
that the whole plant has a stiff and solid aspect ; it forms
tangled masses which gradually decay into a layer of cal¬
careous matter very like tufa.
Such deposits have been exposed to view by the drainage
of small lakes and meres, the marl being useful for agri¬
cultural purposes. Sir Charles Lyell states that the marl
in the Scotch lakes is almost entirely composed of the
shells of mollusca, chiefly Limnœa, Planorbis, Valvata, and
Cyclas, some of them being in a decomposed and pulveru¬
lent state, and some retaining their original form.' Cyprides
and the remains of Chara are also intermixed, and the
whole is compressed into a marl which splits up into thin
layers.
Lyell observes that the inflowing waters must bring a
constant and copious supply of calcareous matter, other¬
wise no tufa or shell-marl is formed. All the Forfarshire
marls occur in lakes which are supplied by springs con¬
taining carbonic acid and carbonate of Mme ; but round
Loch Fithie there are no springs, and in it there is no marl,
though it is surrounded by such deposits, and in every
other respect the site is favourable to the accumulation of
shell-bearing animals. The Charœ, too, are most calca¬
reous in waters free from mud and strongly impregnated
with lime.
^ " Principles of Geology," vol. ii. p. 565.
CHAP. XIII. J
LACUSTRINE DEPOSITS.
239
Large deposits of marl are found in the old meres of the
Cambridgeshire Pens, and Mr. S. B. J. Skertchly attributes
their formation principally to the decay of Cha/rce, which
still abound in the neighbouring dykes.^ Shells fonn,
however, a considerable portion of the mass, and there are
occasional lines of peaty matter. An analysis of the marl
in Whittlesey Mere showed that, after subtracting the
moisture and organic matter (chiefly due to peat), 87 per
cent, of the residue was pure carbonate of lime, 7 per cent,
was an admixture of sandy matter, and the remaining
6 per cent, consisted of sulphate of lime, oxides of iron and
alumina, and carbonate of magnesia in very small quantities.
A small lake in New Jersey, called Milk Pond, is de¬
scribed as being entirely surrounded by a thick deposit of
shell-marl, which seems to cover the whole basin of the
lake. It is perfectly white, and is mainly composed of
bleached shells, and its thickness is known to be more than
twelve feet.
Siliceous.—Deposits, consisting largely of silica, are
formed in some lakes by the agency of minute vegetable
organisms called Diatoms, which are able to secrete and
appropriate the silica dissolved in the water, just as the
molluscs secrete the carbonate of lime to form their shells.
Diatoms consist of simple cells, either single, or united in
a linear series, and each is enclosed in a siliceous case or
shell ; these cases are often elegant in shape and delicately
sculptured on the surface. They are only visible under
the microscope, but compensate for their minuteness by
their extraordinary abundance, being so prolific that the
number of individuals derived from a single diatom in one
month would form a bed of silica, 25 square miles in
extent and 20 inches thick (Ehrenberg).
Diatoms are found in the sea, as well as in bogs, stag¬
nant pools and lakes, and thick deposits are sometimes
formed by the accumulation of their siliceous cases, or
frustules. Such is the berg-mehl (or mountain-meal) on
the shores of certain lakes in Sweden. This is of such a
fine, floury consistency that the inhabitants use it to mix
with flour. Diatom-earth is also used to make dynamite
'■ " Geology of the Fenland," Geol. Surv., p. 60.
240
dynamical geology.
[sec. ii.
by mixing it with nitro-glycerine, and for the material of
certain paints.
A thick deposit of diatom-earth, or diatomite, was re¬
cently found at Loch Quire in the Island of Skye ; the
loch, in fact, seems to have been nearly filled up with the
deposit ; its marshy surface has been drained, and the
diatomite is cut out in blocks like peat. When fresh cut
it has a greenish colour, but when dried it is white, glis¬
tening, and of a floury consistency. It extends to a depth
of 25 feet, and is being rapidly excavated for the purposes
above mentioned.
A bed of similar diatom-earth occurs near the base of
Fig. 62. Diatoms and Spicules of Spongillse from the Banks
of the River Bann.
the Mourne Mountains, county Down, Ireland. It is de¬
scribed by Dr. Drummond as forming a very light. White
substance, resembing in appearance carbonate of magnesia.
On the banks of the River Bann, in the same county, there
is an extensive stratum of similar earth (see fig. 62). The
siliceous frustules belong to species of Navícula, Gallion¬
ella, Coscinodiscus, Gomphonema, BaciUaria, &c., together
with the spicules of freshwater sponges (Spongillœ).
At Bilin, in Bohemia, there is an ancient deposit of
siliceous material, 14 feet thick, which is entirely composed
of diatoms, and chiefly of a species called Gallionella dis-
tans. Ehrenberg calculated that a cubic inch of the earth
contained about forty-one thousand millions of organisms.
CHAP. XIII.]
LACUSTRINE DEPOSITS.
241
It is known locally as polierschiefer, being used by lapi¬
daries for polishing purposes. Similar stone found in
Italy and elsewhere is called Tripoli.
Ferruginous. The substance known as Bog-iron-ore
was also shown by Ehrenberg to be in some cases the pro¬
duct of a particular Diatom called Gallionella ferruginea,
which forms long threads or filaments consisting of a series
of minute cells. Professor Bailey thus describes their
appearance in the pools near West Point, United States :—
" The bottoms of these are literally covered in the first
warm days of spring with a ferruginous-coloured mucous
matter about a quarter of an inch thick, which, on exami¬
nation by the microscope, proves to be filled with millions
of these exquisitely beautiful siliceous bodies. Every sub¬
merged stone, twig, and spear of grass is enveloped by
them ; and the waving plume-like appearance of a filamen¬
tous Ijody, covered in this manner, is often extremely
elegant."
3. Chemical Deposits.—^Deposits due to chemical
precipitation are not common in freshwater lakes, but
some ferruginous deposits have been formed in this way,
especially in lakes which are fed by streams that rise on
boggy uplands or forest-covered mountains. The rocks of
such regions often consist of minerals that contain iron, so
that there is much iron in the soil, and this is taken up by
the organic acids to form humic and crenic salts that are
carried away in solution. When the water containing
them reaches the lakes, and is exposed to oxidation and
evaporation, the iron is precipitated as a hydrous peroxide,
and forms a deposit of soft brown iron ore (limonite) on
the lake-bottom. This occurs in several lakes in Sweden
Fig. 63. Gallionellae (highly magnified).
B
242
DYNAMICAL GEOLOGY.
[sec. ii.
and Canada, and tlie ore is periodically raked up from the
bottom for commercial use. In Sweden the deposit varies
in thickness from 8 to 25 inches, and when removed it is
said to be renewed by the process above described in about
twenty-five years.
Carbonate of lime seems to be chemically deposited in
some Irish lakes, in the form of a white, mealy substance,
which is thus described by Mr. Jukes : ^ " The beds of the
lakes in the limestone district of Ireland have often a thick
deposit of white mud, which, when dry, is almost like flour
in appearance, and is wholly soluble in acids. It is full of
undecomposed freshwater shells of ordinary living species,
but does not itself disclose any trace of an organic origin.
My friend. Dr. J. Barker, of Dublin, subjected some of it,
at my request, to a careful microscopical examination, but
could discover no trace of organic structure. Aroimd
lakes that have been partially drained, large deposits of
this substance, several feet in thickness, may be seen ; and,
in sounding some of the lakes, I usually found the lead
sank into and came up partially coated \^th this substance.
One some parts of the shorés of the lakes there are accumu¬
lations of small nodular concretionary-looking balls of
about half-an-inch in diameter, which Dr. Allman, of
Edinburgh, told me were a species of NulHpore."
§ 2. Saline Lahes.
The deposits formed in saline lakes are chiefly due to
chemical precipitation of the mineral matters carried in
solution by the rivers which run into them. These streams
may of course also lay down deposits of sand and clay in
the manner already described, but we have only now to
consider the matters in solution.
Whether the lacustrine waters retain them in solution,
depends upon the amount of evaporation to which they are
subjected. The air is always absorbing vapour from
broad surfaces of water, so that the water which flows out
of a lake is always less than the amount which flows in.
Some large lakes have no outlet, even though they are fed
' " Manual of Geology," third edition, p. 384.
■CHAP. XIII.J LACUSTRINE DEPOSITS.
243
by several streams, and sometimes tbe evaporation is
greater than the supply, so that the waters are gradually
shrinking in volume. In such cases it is evident that the
proportion of matter in solution will rapidly increase, since
only pure water is taken out; and when the solution
becomes a saturated one, precipitation will commence, and
•chemically formed deposits will be formed.
The basin of the Great Salt Lake in Utah, U.S., affords
an excellent instance of the gradual desiccation of a large
body of water. The present lake is about 80 miles long
with an average breadth of 80 miles, but Mr. Clarence
King calculates that it had originally a length of about
300 miles, and an extreme width of 180 miles.^ The old
lake was probably at first filled with fresh water, but
climatal changes diminishing the supply, evaporation
prevailed and the area of the lake grew less and less ; the
salts were gradually concentrated and finally precipitated
over part of the area.
It would appear that the first salt to be separated was
carbonate of lime, for calcareous sands and large,deposits
of tufa are found on the uppermost terraces; while an
analysis of the present water discloses the fact that none
remains in solution, though large quantities are annually
brought in by the rivers. The reason of this seems to be
that the carbonate of lime is unable to remain in solution
in the presence of alkaline salts, such as the chlorides and
sulphates of soda and potash. It is, therefore, quickly pre¬
cipitated. These alkaline salts are supplied by the nume¬
rous mineral springs in the vicinity.
The next salts deposited from the diminishing lake were
probably some of the sulphates, and finally, when parts of
its area were separated and further concentrated, deposits
of sodium chloride were formed. On the desert plains
round the lake desiccation products are found, which con¬
sist principally of chloride of sodium (86 per cent.) with
sulphate of lime (9 per cent.), and small quantities of the
sulphates of soda and magnesia. In Nevada there is a
salt-field formed by the evaporation of a lake which at the
^ " Exploration of the 40th Parallel," vol. i. p. 492. See also the
account of the older lake area (Lake BonneviUe) by Mr. Gilbert,
Mon. U.S. Geol. Survey, vol. i.
244
dynamical geology.
[sec. ir.
pei'iod of its greatest saturation occupied a basin about 20
miles in length. It is now a solid field of white salt,
covered with a thin layer of brine in the winter months
which completely evaporates in the autumn. In the middle
the deposit of solid salt is five feet thick, but towards the
west shore this is overlaid by a fine saline mud, in which
beautiful octohedral crystals of salt occur. The solid
mass consists chiefly of sodium chloride (97 per cent.)
with small quantities of carbonate and sulphate of soda.
The Caspian Sea and Lake Aral are also good examples
of large water spaces which are gradually growing salter
by a process of concentration. The water in the open part
of the Caspian Sea is fresh because such enormous quan¬
tities of fresh water are poured into it by the Volga, Ural,
and other rivers, but its volume is slowly diminishing ;
salt lagoons are formed round its shores, and the surround¬
ing deserts are covered with a salt efflorescence. The salts
in solution are the chlorides of sodium and magnesium,
the sulphates of lime, soda, and magnesia, with small
quantities of the carbonates of lime and magnesia. Before,
however, the sodium chloride (rock-salt) can be deposited,
it is necessary that more than nine-tenths of the bulk of
water should be removed by evaporation, and until this
saturation point is reached only small quantities of other
salts will be deposited. When, however, the concentration
has proceeded so far a large deposit of rock-salt will take
place. Thus in the salt lakes about the mouth of the
Volga, a compound of the sulphates of magnesia and soda,
called Astrakanite, is formed in the winter when the eva¬
poration is not great, while in summer, rapid evaporation
causes the deposition of rock-salt.
The Sea of Aral is very salt, especially in the shallow
water near its shores ; and Lake Elton, a small lake in the
desert north-east of the Caspian, shows a still greater state
of concentration, its waters being intensely salt and bitter.
Analysis shows them to contain 30 per cent, of solid matter,
two-thirds of this being magnesium chloride, and the
amount of sodium chloride being very small. This shows
that the water is a kind of mother liquor from which the
sodium chloride has been*precipitated.
The water of the Dead Sea has a similar composition.
CHAP. XIII.J
LACUSTRINE DEPOSITS.
245
containing 26 per cent, of saline matter, of which 15 parts
are magnesium cHoride. This great lake is surrounded
by lacustrine deposits, consisting of marls and shales with
beds of rock-salt and gypsum, reaching to a height of 600
feet above the lake, and attesting its much greater size in
former times. The cliffs of Jebel Usdum, at the south end
of the sea, are described by Professor Hull, as consisting
in the lower part of solid bluish rock-salt which is from
30 to 50 feet thick, and is capped by beds of marl, salt, and
gypsum.'
Lakes fed hy Mineral Springs.—"We have hitherto only
considered the chemical deposits produced by the evapo¬
ration and concentration of ordinary river waters when
poured into lakes. But extensive deposits may be rapidly
formed in lakes without great evaporation, if their waters
are supplied by springs which contain a large amount of
mineral matter in solution.
Such mineral springs are especially abundant in districts
where volcanic action has been rife ; they are often thermal,
and generally impregnated with carbonic acid, so that they
usually hold in solution large quantities of carbonate of
lime. The carbonic acid escapes as a gas, and the calca¬
reous matter is precipitated upon the lake-bottoms in the
form of travertine.
In Sir C. Lyell's " Principles of Geology," a description
is given of the lake of the Solfatara, between Rome and
Tivoli.^ This is fed by a stream of warm water proceeding
from a smaller lake above ; and the water is so saturated
with carbonic acid, that the escape of the gas gives it the
appearance of being in a state of ebullition. Tufa and
travertine are formed in the lake at a very rapid rate. The
principal buildings of ancient and modern Rome are built
of travertine obtained from the quarries of Ponte Lucano
in the same district, where there has evidently been a lake
at some remote period in which this deposit has been
fonned.
Travertine derives its other name of Tibur-stone from
the town of Tivoli (ancient Tibur), which stands on an
, ' "Mount Seir and "Western Palestine," Richard Bentley and
Son, 1885, p. 131.
^ "Vol. i. p. 404.
246
dynamical geology.
[sec. 11.
enormous mass of this rock. The walls of the chasm below
the cascade of the river Anio disclose a magnificent section
of tufa and travertine in horizontal beds for a depth of
400 or 500 feet. Lyell observes ; " There can he little
doubt that the whole of this deposit was formed in an ex¬
tensive lake which existed at the close of the period of
volcanic activity, by which the lavas and tufPs of the
Roman territory were formed. The external configuration
of the country has since been greatly changed, and the-
Anio now throws itself into a ravine excavated in the
ancient travertine. Its waters give rise to masses of cal¬
careous stone, scarcely, if at all, distinguishable from the
older rock."
Lahes resulting from the Isolation of Sea-Water.—When¬
ever portions of sea-water are separated by any means from
the open sea so as to form isolated lakes or lagoons, the
solution quickly becomes concentrated by evaporation, and
precipitation takes place.
Sea-water contains about 3^ per cent, of solid matter in
solution, that is to say, every 100 parts of sea-water contain
96| parts of pure water and 3| of mineral matter. In water
from the English Channel the actual amount is 3-525, and
in the Mediterranean 3-765.
When the solid matter is dried and analyzed it is found
to consist of certain salts in the following proportions : ^
Chloride of sodium (common salt)
77-758
Chloride of magnesium
10-878
Sulphate of magnesia
4-737
Sulphate of lime
3-600
Sulphate of potash
2-465
Bromide of magnesium
-217
Carbonate of lime
-345
100-000
The smallness of the quantity of carbonate of lime to be
found in sea-water, compared with that in almost all rivers,
is a point that will be discussed in a subsequent chapter.
Another substance occurring in minute proportions in
rofessor Dittmar, " ' Challenger ' Report
CHAP. XIII. j LACUSTRINE DEPOSITS.
247
sea-water is silica. Forchhaminer found it in all the
specimens of sea-water which he analyzed, the mean pro¬
portion being 9 in 100,000 parts of water.
When a quantity of sea-water is isolated and evaporated
the point of saturation for sulphate of lime is much sooner
reached than that for chloride of sodium ; the former re¬
quiring only 37 per cent, of the water to be removed, and
the latter 93 per cent. Gypsum, therefore, must always be
deposited before rock-salt, and it is possible for this depo¬
sition of gypsum to take place without the point of satura¬
tion for rock-salt being attained. This may be the reason
why, though the sea contains twenty-one times as much
so¿um salt as it does gypsum, that the latter more fre¬
quently occurs as a mineral deposit than the former, though
it is not often found in such massive beds.
A good instance of the formation of gypsum beds from
the concentration of sea-water is described by Professor
Dana, as occurring in the dried-up lagoon of a coral island
called Jarvis Island in the Pacific Ocean.^ The flat surface
of the central basin is covered with a deposit of guano, and
underlying this is a stratum of sulphate of lime, frequently
2 feet thick, resting upon a bed of coral, sand, and shells.
This deposit of gypsum is probably to be explained by the
gradual elevation of the island, during which the lagoon
waters were partially evaporated, but replenished from
time to time by an influx from the sea, so that for a long
time the condensation was not sufficient to precipitate
chloride of sodium. Eventually, however, the whole was
dried up, and salt was deposited, for around the lowest
portion of the basin are incrustations of gypsum and
common salt, ripple marks, and similar evidences of the
gradually disappearing lake. Much of the salt may have
been washed out by rain. Similar deposits of gypsum
occur on many other elevated lagoons among the Pacific
islands.
A good instance of the production of rock-salt by the
evaporation of sea-water is presented by the Bitter Lakes
of the Isthmus of Suez. Before the construction of the
Suez Canal the surface of these lagoons was far below the
\
' Dana's "Coral Keefs," 1875, p. 251.
248
dynamical geology.
[sec. ii.
level of the Eed Sea, and the evaporation of their waters
had produced a bank of salt 66,000,000 square metres
(16,000 acres) in extent, composed of layers which were
5 to 25 centimetres (2 to 10 inches) in thickness. It would
appear that the lakes had been inundated from time to time
by the waters of the Red Sea, while in the intervals between
these incursions the evaporation and concentration was suffi¬
cient to precipitate a layer of salt, and so in time the large
deposit above mentioned was accumulated.
Similarly in the limans of Bessarabia, on the Black Sea,
which dry up in summer, we have the formation of salt beds
going on before our eyes.
One of the salinas of South America is thus described by
Mr. Darwin ;—" The mine consists of a hard stratum, be¬
tween 2 and 3 feet thick, of the nitrate, mingled with
a little of the sulphate of soda, and a good deal of common
salt. It lies close beneath the surface, and follows for a
length of 150 miles the margin of a grand basin or plain.
This, from its outline, manifestly must once have been
a lake, or more probably an inland arm of the sea, as may
be inferred fi'om the presence of iodic salts in the saline
stratum. The surface of the plain is 3,300 feet above the
Pacific."
CHAP. XIV.]
MARINE DEPOSIT.S,
249
CHAPTEE XIV.
MARINE DEPOSITS.
IN describing the operations of a river, we bave followed
tbe course of the detritus transported by its current,
and have seen, that though this is temporarily deposited at
certain points along the river-valley, yet most of it is
eventually moved on again and triturated into smaller and
smaller particles. Sediment is thus continually brought
down from higher to lower levels, and never finds a per¬
manent resting-place until it is carried into a lake or into
the sea. Even delta-mud is liable to removal, and from
its mouth the river is constantly discharging a cloud of
sediment, which is carried away by the tidal currents and
spread over the sea-bottom.
Excepting, therefore, the small portion which is inter¬
cepted by lakes, the sea is the ultimate recipient of the
materials carried down by rivers. To this is added the
detritus which the sea-waves erode from the coast, and the
whole is carried by the marine currents tiU circumstances
determine its deposition. Some of it is carried along the
coast and sorted by the action of waves and currents tiU it
is thrown up and deposited in bays and inlets. The rest
of it is carried out to sea, and dropped at a greater or less
distance from land, according to the fineness of the sedi¬
ment and the strength of the tidal current, but it is onlv
the very finest material which is carried more than lOi)
miles from land.
We must also remember that besides this visible sedi¬
ment there is much invisible material dissolved in the
water, and that this latter also is poured into the sea.
The lime is used by millions of marine creatures in
building up their shells or stony structures, and the silica
is used in the same way by certain lowly animals and
plants. On the death of these organisms the hard parts
250
DYNAMICAL GEOLOGY.
[sec. II,
remain and accumulate, either on the sea-ñoor or against
its margins. Corals, Mollusca, and Echmoderms are the
limestone builders of the present day as they have been in
all past time, and it is essential that the student should
form an accurate conception of the conditions under which
such rocks have been formed. For the purpose of making
the account of these as clear as possible we shall describe
the deposits under the following heads :—
1. Shallow-water Deposits . ' | g" ^g"nic.°^^
2. Deep-water Deposits . . . | ^;
1. Shallow-Water Deposits.
In the present chapter shallow-water deposits formed
of inorganic current-borne detritus derived from the waste
of the land will alone be considered.
Assortment of Material.—Let us commence with the
case of a simple coast-line like that of the north coast of
Spain, bordering the Bay of Biscay, or that of the west coast
of South America, where for many hundred miles no large
rivers enter the sea, and where the conditions of deposit
are consequently reduced to their simplest form. On such
coasts the short and rapid torrents from the mountains
carry material to the sea, and the waves quarry more from
the cliffs, and these are regularly sorted by the tidal
currents. Thus for several thousand miles along the
coasts of Peru and Chili, there is a perpetual rolling of
shingle along the shore, parts of which are incessantly re¬
duced to the finest mud by the waves and swept out into
deep water by the tides and currents. Off this coast are
successive deposits of shingle, sand, and mud, disposed
in regular longitudinal bands, the coarser portions always
nearest to the shore, and the finer sediment furthest away
from it.
The same is the case along the eastern coast of Pata¬
gonia, and the soundings taken between Santa Cruz and
the Falkland Islands in H.M.S. " Beagle," are mentioned
by Mr. Darwin ' as a good instance of this assortment of
^ " Voyage of the ' Beagle,' 1876."
CHAP. XIV.] MARINE DEPOSITS.
251
material, tte size of tlie stones decreasing regularly witlk
the distance from shore :—
""Sr" XJl Size of material.
2 to*4 11 to 12 Pebbles as large as walnuts with.
others of smaller size inter¬
mixed.
4 to 7 17 to 19 Pebbles about the size of hazel
nuts.
10 to 11 23 to 25 Pebbles about the size of peas.
12 30 to 40 Pebbles of an inch diameter.
22 to 150 45 to 65 Coarse to fine sand.
Let us next take the case of a small river with a rapidl
current carrying detritus of all sorts, from small stones
and pebbles to fine mud, according to the volume of water,,
■ and discharging them into a sea where there is no strong
cross-current. As the river enters the sea it will first de-
Pig. 64. Lenticular Deposits on a Sea-bottom.
s, s, the surface of the sea. a, pebble-beds. 6, sand, c, mud.
posit the pebbles, next the smaller stones and coarse sand,
farther out fine sand, and last of all, silt and mud. This
order of succession is always maintained except where in¬
terrupted by local currents and influences. The width
and extent of these deposits will, however, vary from tim©
to time ; when a river is in flood the coarser materials will
be carried out to a greater distance, so that pebbles and
shingle will be deposited over a tract of sand, and sand will
invade the region of mud : again, when the river is low,
clay or mud will settle down nearer shore, as it did before
the flood-time, and cover up the outer beds of sand. In
this way wedge-shaped or lenticular deposits of the various
materials will be formed and interbedded with each other,,
as indicated diagrammatically in fig. 64.
There will be a further assortment of material according
to the differences in the shape and specific gravity of th©
particles composing the muddy sediment. This subject
•252
DYNAMICAL GEOLOGY.
[sec. ii.
has been investigated by Mr. Babbage, ■who supposes the
-case of a river, the mouth of which is 100 feet deep (and. a
current of 2 miles per hour), delivering four varieties of
fine detritus into a sea which has a uniform depth of 1,000
feet (166 fathoms) over a great extent, which sea is tra¬
versed by an ocean current moving in the same direction
-and with the same velocity.
He takes for granted that the four varieties of sediment
are such as, from their size, shape, and specific gravity,
would fall thi'ough stUl water, the first 10 feet per hour,
the second 8 feet, the third S feet, and the fourth 4 feet.
The combined effect of the downward motion of the de¬
tritus and the onward motion of the water, would then
bring the first variety to the bottom of the sea, at a dis¬
tance of 180 miles from the river's mouth, and strew it
■over a space 20 miles long ; the second variety would only
begin to reach the bottom 225 miles from the river's mouth,
and would be spread over 25 miles, and so on, as in the
following table :—
No.
Velocity of
fall per hour.
Nearest dis¬
tance of deposit
to river mouth.
Length of
deposit.
Greatest dis¬
tance of deposit
from river
mouth.
Feet.
Miles.
Miles.
Miles.
1
10
180
20
200
2
8
225
25
250
3
5
360
40
400
4
4
450
50
500
We should thus have, proceeding from the same river, and
poured into the sea either simultaneously or at different
times, four different and widely separated patches of mud
•or clay on the sea-bottom.
It must not be supposed, however, that all the detritus
poured into the sea by rivers, or obtained by coast-erosion,
is distributed in this regular manner. The sea-shore is
not always bordered by a shingle-beach, neither is the finer
sediment invariably carried out to a distance from land.
CHAP. XIV.J
MARINE DEPOSITS.
253
Many rivers never carry anything coarser than sand or silt
into the sea, even when floods have occurred in the higher
parts of their valleys. Other rivers terminate in estuaries,
where they only deposit the finest mud. Again, the mate¬
rials derived from the erosion of clifEs are often carried
laterally along the shore to great distances ; the set of the'
currents varies with the height of the tide, and sometimes
an undercurrent will set in one direction, while there is an
overflow in a different direction, so that much even of the-
finer sediment may be carried along the shore and depo¬
sited where the currents slacken in bays or inlets.
Thus there are tracts along every coast-line where the
land is gaining upon the sea in consequence of the restora¬
tion of some of the material which has been torn from it
elsewhere. Shingle-beaches, river-bars, and sand-banks
are being formed along shores which do not supply the-
material of which these deposits are composed, sheltered,
bays and estuaries are being silted up with materials intro¬
duced by marine currents, and the overplus of sand throwm
up by the waves is often blown by the wind into sand hills
which serve to protect the newly formed and low lying
land from the effects of storms and high tides.
Shingle Beaches.—We have seen that the coarser
portions of the débris won from the land are generally left
or washed up on the shore near high-water mark, and form
the shingles or pebble-beaches of which mention has already
been made.
Attention to the changes of a shingle-beach will soon
show that besides the moving of the stones backwards and
forwards with every tide there is also a general lateral
movement along the shore. This is due to the set or in¬
clination given to the waves by the prevalent winds and
currents ; the breakers thus come to have a slightly oblique-
action ; they do not drop a pebble exactly where they picked
it up, but are constantly shifting the pebbles a little fufther
on in one direction. The beach, therefore, considered as a.
whole, may be said to travel in that direction, until it is
: arrested by some point of land, or by some artificially
constructed barrier.
It is possible also for stones and pebbles to be carried in
various directions through the mediation of sea-weeds.
'254
dynamical geology.
[sec. ii.
Mr. G. H. Kinahan ' has observed certain beaches on the
Galway coast, where very large rounded stones occur,
and found that the quantity of these was increased after
storms, and that some of the stones must have travelled
through water of 15 fathoms in order to reach the beaches.
On investigation he discovered that similar stones were
scattered over the sandy bottom of the sea outside, and
that laminaria and other sea-weeds grew luxuriantly on
them, often having fronds 20 feet long. These fronds
made the stones so buoyant, and gave such a surface for
waves and currents to act on, that though unmoved in
calm weather they were transported landwards during
storms, and were eventually thrown up on the beach,
where the weeds decayed and the stones were added to
the shingle.
On the east coast of England sand and shingle travel
from north to south, because a strong current sweeps south¬
ward along the shore. On the south coast beaches travel
eastward, in consequence of the prevalent winds and
strongest currents both setting from the S.W. If a pier
or groin be erected anywhere on this coast a bank of
shingle will be gradually accumulated on its western side.
Long shingle beaches are formed at certain points, and
nearly all the pebbles composing them have been ob¬
tained from the clifEs and shores to the westward of that
-place.
The Chesil Bank, on the coast of Dorset, is a good in¬
stance of such a beach. It commences near Burton Brad-
stock, and extends as far as the Isle of Portland, a distance
■of 17 miles. For the first 5 miles it is banked against the
shore ; from Abbotsbury onwards there is a narrow channel
or estuary, called the Fleet, on its inner side, this section
of its length having a height of 20 to 30 feet above high-
water mark, and a breadth of about 500 feet. The last 5
miles form a bar or isthmus connecting the Isle of Port¬
land with the mainland, and at the S.E. end of this, its
height is 40 feet and its breadth about 600 feet. It is a
remarkable fact, that although the pebbles composing the
Leach have all come from the westward, those at the N.W.
^ "Proc. Roy. Irish Ac.," 2nd Ser. vol. ill. p. 202. -
€HAP. XIV.J
MARINE DEPOSITS.
255
■end are very small, and they gradually increase in size
towards the S.E. The reason of this is probably to be
found in the greater size of the waves which break upon
the south-eastern end, while nearer the head of the bay
the waters are quieter and more protected from storms.
The larger waves throw up the larger stones, and thus
the material coming from the west is sorted according
to size and weight along the whole length of the Bank.
Biurst Castle Bank is another good example of a shingle-
beach. This juts out from the Hampshire coast, across
the western entrance of the Solent, thus forming a bar
which is about two miles long, 70 yards broad, and 12 feet
high. It consists of rounded flints derived from the waste
of the gravel-capped cliffs to the westward.' Other beaches
formed in the same way of material coming from the west
occur at Eastbourne and Dungeness.
Sand-flats and Sand-banks.—Along all the more open
parts of a coast-line sand is generally accumulated ; where
there is a shingle beach the sand forms a flat below it, and
is only exposed at low water, but where there is no shingle
it forms the foreshore also, being driven up into ridges liy
the storm-waves, and often blown into sand-hiUs by the
wind (see p. 200).
Open bays are favourable situations for the accumula¬
tion of sand, the material being brought into the bay by
the tidal current and dropped at high tide, while the ebb
flow in many cases only carries back the flner sediment and
leaves the sand above low-tide level. This sorting action
is assisted by the waves, which keep the surface of the
sand-flat in a state of constant oscillation, both during the
rising and falling of the tide. During storms and high
winds this wave-action extends to deeper water, as is
proved by the casting up of molluscs and other animals
which are known to live on the bottom in moderate
depths. Wherever tidal currents act strongly they keep the
materials on the bottom in a state of movement, but where
they spread over wide and shallow surfaces these materials
accumulate to a greater or less extent. It is in this way
that the floors of so many of our bays are paved with
sand.
' Lyell's " Principles of Geology," vol. i. p. 531.
256
DYXAMICAL GEOLOGY.
[sec.ig
Morecambe Bay on the Lancashire coast is a good
example of a bay which is being gradually filled up with
sand ; in this case the sand is supplied partly by the rivers
flowing into the bay (the Lune, the Kent, and the Leven),
and partly by the flood-tide entering the bay, by which the
whole supply is distributed. Including the Lancaster and
Fleetwood Sands, there are about 160 square miles of sand
and silt exposed at low water over this tract. The depth
of the deposit is unknown, but is probably at least 40 or 50
feet in many places. Shifting quicksands occur within it, in
which a coach and horses have been swallowed up.
As a case of accumulation under different conditions we
may mention the sands at the mouth of the Thames. The
material for these has been brought entirely by the tidal
currents, and is probably derived from the waste of the
Norfolk and Suffolk coasts. Part of it is arrayed at the
mouth of the estuary in long sand-banks, with deep
channels between them, but a great quantity is thrown up
on the Essex coast, where it forms the large sand-flats
known as the Maplin and Foulness Sands, of which an
area of about 40 square miles is exposed at low water.
With regard to the depth of sand on these banks, Mr.
T. H. Tizard ^ says " it is probable they are upwards of
60 feet thick, for channels of that depth have opened out
across the sands and again closed up, so that the bank has
been dry at low water where 60 feet formerly existed ; and
the Goodwin Sands in the Downs, which have been bored,
proved to be 80 feet thick."
The sands of Pegwell Bay, between Sandwich and Eams-
gate, are another good instance of sands carried and de¬
posited entirely by marine currents. In this case the sand
travels from the south, and is washed up into the bay
which is formed by the projecting cliffs of the Isle of
Thanet.
Silt and Mud-flats.—These appear to be formed in two
ways. Some are due to the formation of shingle beaches
or sand-bars across the mouth of an estuary, causing the
water inside to spread over a wide area, in which the sedi¬
ment brought down by the river and that brought in by
' " Nature," April, 1890, p. 539.
CHAP. XIT.]
MARINE DEPOSITS.
257
the tide is spread out. The marshes of the Tare have
been formed in this way.
" In the time of the Saxons the valley of the Tare was
a great estuary, and extended as far as Norwich, which
city is represented, even in the thirteenth and fourteenth
centuries, as ' situated on the hanks of an arm of the sea.'
The sands whereon Yarmouth is built, first became firm
And habitable ground about the year 1008, from which
time a line of dunes has gradually increased in height and
breadth, stretching across the whole entrance of the ancient
estuary, and obstructing the ingress of the tides so com¬
pletely, that they are only admitted by the narrow passage
which the river keeps open, and which has gradually
shifted several miles to the south."
" By the exclusion of the sea, thousands of acres in the
interior have become cultivated lands; and, exclusive of
smaller pools, upwards of sixty fresh-water lakes have
been formed, varying in depth from 15 to 30 feet, and in
extent from 1 acre to 1,200. The Tare, and other rivers,
frequently communicate with these sheets of water ; and
thus they are liable to be filled up gradually with lacus¬
trine and fiuviatile deposits, and to be converted into land
covered with forests." '
Other marshes and mud-flats are formed off open shores
entirely by material brought in from the sea ; the condi¬
tions being apparently such as will cause the current
setting along the coast to slacken gradually, so that it first
drops the sand, and is only able to carry fine sediment
shoreward over the sand-fiats. This process can be watched
in operation along the shores of the Wash. The strong
current from the north brings sand and silt into the bay of
the Wash, and the waters of every tide bear along a cer¬
tain amount both of the coarser and finer material. The
former is naturally deposited before the latter, and forms
sand-banks and sandy flats which extend up to the limits
of ordinary high-water mark, but are constantly varying
in their position and conformation. Inside these sand-fiats,
are mud- or silt-flats which are only covered at spring-tides.
When the water reaches these higher flats, the force of the
' Lyell's " Principles of Geology," vol. i. pp. 516, 517.
s
258
dynamical geology.
[sec. ii.
tide is slackening, and the current is never strong : the
muddy waters slowly spread over the nearly level surface,
and much of the sediment which they carry is deposited
during the slack of the tide. This deposit remains where
it rests, for the slight movement of the ebb is not sufficient
to disturb it, although it is capable of carrying away such
sediment as remains in suspension.
The tract of shore, therefore, which lies between the
high-water marks of ordinary tides and spring-tides, is
slowly raised by the thin sheet of mud which is spread
over it at every spring-tide; every such sheet or layer
forms a separate lamina, and the whole accumulation is a
deposit of laminated silt.
When the surface of the flat has thus been raised to a
certain height, the glass-wort (Salicorina Tierhacea), locally
known as the samphire, begins to grow upon it, and facili¬
tates the further deposition of silt, which now rapidly accu¬
mulates. The surface becomes gradually covered with sea-
thrift and other marsh-plants, and passes into the condi¬
tion of " green marsh," which is only overflowed at very
high spring-tides. Mr. Skertchly observes that the " sam¬
phire marsh " is always distinguished from that which,
having become covered with verdure, is called " green
marsh " ; also that the two tracts occupy different levels,
the samphire marsh being only 8 6 feet above Ordnance
datum, and the green marsh averaging 11 feet above the
same datum.
The Marsh-land of East Lincolnshire, which extends from
the mouth of the Humber to the Wash southward for a
distance of 35 miles, with an average width of 8 to 4 miles,
has been formed in the manner above described. Beds of
peat containing branches and trunks of trees are inter-
bedded with the marine silts and clays, and mark periods
when deposition was interrupted ; but the series is essen¬
tially marine, and is from 30 to 40 feet thick near the
coast. The sediment of which it is composed has been
derived partly from the erosion of the coast of Holdemess,
and partly from the supply brought down by the Humber.
The Marsh-land is not now being added to except at and
north of Saltfleet, while the southern part of it from
Mablethorpe to Wainfleet was once more extensive than
CHAP. XIV.J
MARINE DEPO.SrrS.
259
it is now, having at various times been reduced by inroads
of the sea.
The Marsh-land passes southwards into the wide level
of the Fen-land, which is not a delta (as sometimes stated),
but a sdted-up bay. The silts and clays of the Fens are
not fluviatile, but marine deposits ; they are a continuation
of the marsh beds, and have been formed in the way above
described. Mr. Skertchly states^ that in some places a
breadth of 3 miles of land has been formed since the
Eoman occupation, and that the process is now going on.
About 55 square miles were thus added to England
between the second and seventeenth centuries, i.e. in 1,500
years; assuming, therefore, that the rest of the Fen-land
has been accmnulated at the same rate, and estimating the
area of silt and clay land at 550 square miles, the time
occupied in its formation will have been 15,000 years.
Silting up of Bays on the South Coast.—It would
be easy to indicate the tracts along the eastern and southern
coasts of England where similar additions have been made
to the land, just as in Chapter X. we mentioned those places
where land had been lost by the action of the sea ; but it
would occupy too much space to describe all these in
detail, and it will suffice to mention a few of the more
notable instances of accretion that occur on the south
coast.
The first of these is the union of the Isle of Thanet to
Kent in historic times. " The isle of Thanet was, in the
time of the Romans, separated from the rest of Kent by a
navigable channel, through which the Roman fleets sailed
on their way to and from London. Bede describes this
small estuary as being, in the beginning of the eighth
century, three furlongs in breadth ; and it is supposed that
it began to grow shallow about the period of the Norman
Conquest. It was so far silted up in the year 1345, that
an Act was obtained to build a bridge across it ; and it
has since become marsh-land, with small streams running
through it.
This silted-up channel widens out eastward into a tract
of marsh-land which extends along the border of Pegwell
' " Geology of the Fenland," Mem. Geol. Surv., pp. 9, 182.
260
DYNAMICAL GEOLOGY.
[sec. ii.
Bay from Minster to Deal. It is in fact a silted-up
portion of that bay, and the old Eoman port of Eitupium is
now a mile and a half from the shore.
The rich level tract called Eomney Marsh, lying between
Hythe and Hastings, is the site of a silted-up bay ; mea¬
sured along the coast, its frontage is about 25 miles in
length, and its area is about 100 square miles. The sea is
now excluded from the marsh, partly by natural barriers
of blown sand and shingle, partly by artificial banks, but
the greater part of its surface is below the level of high
tide. The marine and estuarine deposits filling this tract
are sometimes 70 feet deep, the lower part being always
sand with marine shells, and only the upper 10 or 15 feet
consisting of silt and clay with layers of peat. The marsh
has received great accession within historic times. The
town of Eye was once destroyed by the sea, but it is now
two miles distant from it.
In Sussex, between Hastings and Eastbourne, there is a
considerable area of marsh, which doubtless occupies the
site of a silted-up bay. In Pevensey Level the general
succession of deposits is—(1) Vegetable soil; (2) Clay,
with fresh-water shells ; (3) Clay, with marine shells.
The Lewes Levels, between Newhavenand Lewes, occupy
the site of an estuary which had been silted up within the
last 800 years. The usual succession of beds found here
is described by Dr. ManteU as follows, the total depth
being from 30 to 36 feet :—
1. A bed of peat 5 feet thick, enclosing trunks of trees.
2. Blue clay, containing fresh-water shells.
3. Blue clays, containing a mixture of fresh-water and
marine shells.
4. Blue clay, with marine shells only, and skull of
Narwhal.
5. At bottom a bed of white marly clay.
The contents of these deposits give the key to the whole
history of their formation. The clay with marine shells
indicates the period when the sea had free access to the
valley ; subsequently a shingle bar was formed across its
mouth, blocking out the salt-water, and causing the for¬
mation of a marsh. The gradual change from salt to
fresh-water conditions is plainly shown by the increasing
CHAP. XIT.J MAKINE DEPOSITS.
261
proportion of fresh-water products, till at length it became
a land surface, and supported a thick growth of forest. ^
Selsey, as its name implies, was once an island, and was
separated from the mainland by a shallow estuary, the
site of which is now occupied by Thorney Marshes and
Pagham Harbour. The old historian Bede (a.d. 731),
describes the place " as encompassed by the sea on all
sides except the west, where is an entrance about the cast
of a sling in width ; which kind of place is by the Latins
called a peninsula." By the time of the Conquest, how¬
ever (a.D. 1060), it appears to have been completely joined
to the mainland.
Poole Harbour, on the coast of Dorset, has been silted
up in still more recent times. Mr. Brannon, who examined
and reported on the harbour in 1859, has traced the fol¬
lowing sequence of causes. He says : " As long as the sand
cliffs to the east of the harbour stood southward of certain
lines of bearing with the Isle of Purbeck, all the sand worn
from them was carried out to sea and deposited in the
Channel. So soon, however, as these cliffs had been cut
back within these lines of bearing, the ebb currents formed
an eddy outside the harbour, and in it some of the sand
was deposited. This sand-bank (the Hook Sand) does not
appear to have existed before the reign of Henry VIII.,
and when it had so increased as to rise above low-water
mark, the wind drove up sand on to the shore, and sand¬
hills were formed across the mouth of the harbour, leaving
only the present narrow entrance." He thinks the silting-
up of the harbour began only about 200 years ago, but at
first the changes were very slight, and were scarcely
noticeable up to the end of the eighteenth century. The
material is brought in from the Hook Sand by the tide,
and since the beginning of this century the flood-tide has
taken in very much more than the ebb carried out, and
what was once a fine harbour is being rapidly converted
into a marine marsh.^
Deposition increased by Depression.—It is evi¬
dent that when any part of a bay, or estuary, is silted up
to the level of high-water mark, no farther deposition can
' Mantell's "Wonders of Geology," seventh edition, p. 62.
^ See "Geologist," vol. iii. p. 430.
262
DYNAMICAL GEOLOGY.
[sec. ii.
take place over its surface : the material brought down
by the streams is carried on and deposited outside that
already accumulated, and so the newly-formed land in¬
creases by the addition of matter to its margins, as already
explained in the case of deltas.
It is clear also that if the area of deposition is being
raised, the silting-up will be accelerated, for the amount
of matter deposited over any one spot will be less than if
the area were stationary. On the contrary, if the whole
area be permanently lowered a few feet, mud and silt will
again be thrown down and will continue to accumulate
until it raises the surface to the former level, and thus
a greater amount of matter will be deposited over a given
spot. Continued subsidence will allow of equally continued
deposition, and in this manner the thickness of deposits
may be indefinitely increased.
Deposits in Shallow Seas.—Much of the material
gained from the land is carried seaward by the retiring
tide and by the currents which set along the shores, and
is eventually deposited at a greater or less distance from
the land, according to the strength of the current and the
fineness of the material.
The seas surrounding our own islands are exceptionally
shallow, the North Sea being in few places more than
50 fathoms deep, and there are only two parts of the Irish
Sea which reach that depth. From the deposits which are
now being formed in these seas, we may obtain a good
idea of the manner in which the materials worn from the
surrounding shores are disposed over their bottoms. The
Admiralty charts, like all other good charts, indicate by
letters the nature of the bottom as well as the depth ; and
by colouring these charts with different colours, according
as the bottom consists of sand or mud, we get a very
instructive maji of the deposits now being formed.
Sand is the prevalent deposit in all these shallow seas.
Thus the floor of the North Sea may be described as a vast
expanse of sand interrupted only by a few tracts of mud.
The chief of these mud tracts is a rather large space to
the north-east of Scotland, and beyond it there is an area
of clayey bottom, which probably dates from the Glacial
period, and is not a modem deposit. In the middle of the
CHAP. XIV.J
MARINE DEPOSITS.
263
Nortli Sea, tetween Denmark and the Dogger Bank, there
is a considerable area of muddy sand and silt, but this is
entirely surrounded by sand.^
In the English Channel sand also predominates, but
much of it is shell sand, -which -will be described under the
head of organic deposits. There are some irregular tracts
of gravel and shingle which occur at various depths, and
most of them are probably ancient deposits formed under
different geographical conditions than those which now
prevail.' There are also several tracts of bare rock, evi¬
dently spaces swept clear of deposit by the strong currents
which set in and out of the Channel.
Outside the mouth of the Channel there is a submarine
plateau which extends for 200 miles west of Cornwall, and
for the same distance south of Ireland, and is everywhere
covered by less than 100 fathoms (600 feet) of water.
Over the greater part of this large area sand of varying
degrees of coarseness prevails, with here and there a patch
of gravelly or stony bottom, and a few tracts of mud.
One of these last lies off the mouth of the Bristol Channel,
and about midway between Cornwall and Ireland, its
existence apparently depending more on the influence of
currents than, on the depth of water. It is entirely sur¬
rounded by an irregular tract of fine sand and silt, which
is prolonged eastwards toward the mouth of the Bristol
Channel, and southward to encircle another tract of mud,
which lies to the west of the Scilly Islands. The disposi¬
tion of these deposits is represented on the map, fig. 65,
the scale of which is about 80 miles to an inch.
There are one or two other small patches of mud and
muddy sand, but all the outer part of the plateau, which
is between 70 and 100 fathoms deep, has a bottom of sand
and shelly sand. Such a distribution of material is an
exception to the rule that the fineness of the sediment
increases with the distance from land, and is evidently the
outcome of some special conditions. We should have
oxpected that at a certain distance from the shores of
Ireland and Cornwall fine sand would everywhere be
^ For details see Delesse, " Lithologie du Fond des Mers,"
Paris, 1872.
" See Godwin-Austen, "Quart. Journ. Geol. Soc.," vol. vi. p. 69.
Fig. 65. Scale 32 miles to an inch.
CHAP. XIV.J
MARINE DEPOSITS.
265
found, that further out there would he mud, and that
the mud would extend westward until it gradually passed
into the deep-sea muds and oozes of the Atlantic, which
will he described in the next chapter.
This, however, is not the case, and the anomaly may
perhaps he accounted for partly hy the peculiar contours
of the sea-floor, and partly by the influence of waves of
translation generated in the Atlantic, and affecting a
great depth of water. Mr. A. Hunt has discussed this
matter, and writes : " It seems highly probable that water
displaced by wind pressure and atmospheric pressure over
large areas in the Atlantic enters the Channel in the form
of waves of translation, and there stirs up and places
within the grasp of other currents all silty deposits light
enough to be disturbed. Such removal of mud and silt
would account for the sandy character of the bottom as
evidenced by the charts." ^ The mud so sifted out would
be redeposited nearer shore, and the two tracts in flg. 65
probably lie beneath areas where the surface currents have
a tendency to circulate.
In the Irish Sea a large area of mud commences off the
coast of Heath, and extends northward between co.^ Down
and the Isle of Man for a distance of 60 miles, with an
average width of 24 miles, giving an area of about 1,400'
square miles of mud. It is surrounded by a narrow tract
of muddy sand, which is prolonged northward through the
North Channel, and north-eastward to the mouth of the
Solway Firth. There is also a long tract of muddy sand
off the coasts of Lancashire and Cumberland. Neither
of these tracts depend much on the depth of water.
Returning to the plateau south of Ireland a narrow
strip of sand runs northward, covering the shelving slope
which lies between the south-west of Ireland and the
200 fathom hne, and thence spreading out over a wide
area of the sea-floor between Ireland and the Porcupine
Bank, much of this area being less than 100 fathoms
(600 feet), and all of it less than 200 fathoms. On the
south side of this tract there is a deep submarine bay
which is fllled with fine sand and mud.
^ "Denudation and Deposition by the Agency of Sea Waves,"
printed for private circulation, 1889, p. 27.
266
DYNAMICAL GEOLOGY.
[sec. ir.
From the north of Ireland and outside the west coast of
Scotland and the Hebrides sand largely prevails within
the 200-fathom line, which lies at a distance of from 100
to 150 miles off the main coast. The sand is interrupted
by small areas of mud, by larger tracts of shell sand, and
by occasional patches of bare rock where strong currents
have prevented the accumulation of deposit.
Besides the tracts of mud which have been noticed above
there are some which are directly due to river currents,
and lie opposite the river mouths ; these, as Mr. Lebour
has observed, may be termed submarine deltas} That of
the Thames forms a spread of sandy mud or silt in the
shape of a triangle, the base of which runs from Margate
to Buxey Sand, but is interrupted by sand-banks already
mentioned. The delta of the Seine is very well marked,
and consists of similar materials, but its widest part faces
the sea, and its apex runs up the estuary : a difference due
doubtless to the conformation of the coast. At the mouth
of the Firth of Tay there are mud-banks which have been
recognized by Mr. J. Geikie as the submarine delta of the
Tay.'' The Elbe also forms a clearly marked submarine
delta of mud, which is spread over the sea-floor between
its mouth and Heligoland, and covers an area of about
180 square miles, while on either side the bottom consists
■of sand. The shallower slopes on either side have a sandy
bottom.
Limits of Terrigenous Deposits.—It is evident
from Mr. Babbage's calculation, above quoted, that there
is a limit to the outward extent of the deposits derived
from the erosion of the land. Under the conditions
assumed by him this would be at a distance of 500 miles
from the shore, but these conditions nowhere exist, and
the extension of such deposits is generally much less.
In the flrst place it is assumed that the ocean current
which bears the river-borne detritus onward flows in the
.same direction as the river, viz., directly out to sea ; but
this can seldom be the case, for most marine currents,
whether in seas or oceans, sweep along the coasts or set
outwards in an oblique direction from the shore. Sedi-
' " Proc. Geol. Assoc.," vol. iv. p. 162.
' " The Great Ice Age," 1874, p. 520.
CHAP. XIV.J
MARINE DEPOSITS,
267
ment, therefore, which is carried by such currents will not
reach so great a distance from land as 500 miles, for sup¬
posing that the current sets outward from the river's
mouth at an angle of 45° with the trend of the shore,
sediment carried 500 miles in a linear direction will only
be 350 miles from land when deposited. Secondly, it is
probable that such currents are superficial, and do not
retain their surface velocity down to any great depth.
The actual river-current is certainly superficial, and as
soon as the sediment has sunk through that it will be
carried by the marine currents, which may have a different
velocity, and the set of which may be in any direction.
As a matter of fact, the disposition of the currents round
continental coasts is such that terrigenous material is
seldom carried to more than 150 or 200 miles from land.
The exceptions are off the mouths of the largest rivers of
the world, such as the Amazon, the Mississippi, the Indus,
and the Ganges. Blue muds are found at great distances
from land in the Arabian and Indian Seas, but they are of
peculiar composition, and extend to great depths, so it will
be more convenient to describe them under the head of
deep-water deposits.
Marine Deposits due to the action of Ice.—In
considering the manner in which the products of ice-
erosion are deposited on the sea-bottom, it must be remem¬
bered that there are two distinct kinds of ice in the sea,
viz., land-formed ice, and sea-f®rmed ice. The first is
delivered into the sea from the end of glaciers, the second
is produced by the freezing of the sea itself along the coast.
Both carry detritus, but since the conditions of the two
cases differ, it is possible that the deposits resulting from
their action differ also in some degree, though perhaps not
so greatly as might at first sight appear probable. We
shall, however, describe each case separately.
I. Glaciers and Icebergs.—In the regions of Arctic cold
where the snow-line reaches the sea-level, the glaciers
descend into the sea, and large masses of ice are constantly
breaking off and floating away as icebergs. These bergs
often carry stones and blocks of rock which are dropped
on the sea-bottom as the mass of ice gradually melts
away.
268
dynamical geology.
[sec, 11.
The glaciers of Greenland and Spitzbergen have already
been described. Some of them are 2,000 feet thick, and
terminate in high cliffs more than 200 feet in height,
■which sometimes continue like a wall for a distance of 50
or 60 miles in front of the land. From these cliffs gigantic
icebergs are from time to time dislodged, often rising from
100 to 200 feet out of the sea, which involves a thickness
of nine or ten times that amount below water.
We have seen that in the case of inland glaciers the
rock-fragments which they bear along fall on to the ter¬
minal moraine, while the finer detritus is carried away
from the locality altogether by the stream which issues
from beneath the glacier. When, however, a glacier ter¬
minates in the sea, both these kinds of detritus are de¬
posited together in the bay or fiord where the glacier ends.
Sub-glacial rivers occur beneath the inland ice-sheet of
Greenland, just as they do beneath the Alpine glaciers, and
along certain parts of the coast where the ice does not
reach the sea, these streams burst out as turbid torrents
from below the terminal wall of ice. So also do they
escape beneath the glaciers which descend into the gulfs
and fiords, and where the ice is protruded far into the sea,
the muddy water is discharged at a depth considerably
below the level of the water, and the fine powdery mud
sinks to the bottom in a continuous cloud -without being
transported far from the spot.
At the same time the bergs broken off from the glacier
will drop a large part of their freight of stones and boul¬
ders as they fioat away ; much more debris of the same
kind will be contributed by the periodical melting of the
coast-ice ; and all -will be embedded in the mud which is
thus being deposited on the sea-bottom.
Fig. 62 is intended to illustrate the termination of a
glacier in the sea, a representing the glacier, h one of the
bergs detached from it, and c a deposit of clay and stones
on the sea-bottom.
2. Coast-ice.—The nature and action of coast-ice have
been described on p. 184. It was there stated that the
ice-foot becomes loaded -with a great quantity of rock-
débris, much falls on to its surface from the cliffs above,
many stones and boulders are picked up from the shore to
CHAP. XIV.J
MARINE DEPOSITS.
269
■which it is frozen. When summer comes the ice breaks
o£E from the coast and floats away ■with its load of land-
debris, drifting about in the bays and along the coast, but
not often travelling very far out to sea. Sometimes the
floes are driven upon the shore during storms, and packed
to a height of 50 or 100 feet, sometimes they break up
over comparatively deep water, and their freight of mud
and stones is then scattered over the sea-bottom.
The deposits thus formed must be accumulated rapidly
and continuously, the materials are very various and are
dropped indiscriminately upon the sea-bottom, the por¬
tions already deposited are constantly disturbed by falling
fragments, and frequently exposed to the impact and
pressure of ice-floes ; under these circumstances they are
not likely to present the clear lines of stratification which
Fig. 66. Termination of a Glacier in the Sea.
other sediments slowly accumulated and sorted by current
action always present.
Furthermore, this glacial deposit will not be confined to
valleys, but will spread over the whole sea-bottom near the
coast, and ■will cover all its irregularities.
It should be noted, too, that some of the boulders trans¬
ported by coast-ice are carried to lower levels, and others
to higher levels, the latter being especially the case if the
whole coast is sinking. The ice-floes will then be driven
further and further on to the land, bearing with them the
stones and blocks derived from those parts of the shore
which were previously exposed to their action, so that by
this means rock-fragments may be carried upward in
course of time to levels far above that from which they
were originally detached.
Mr. Darwin long ago explained this transportal of
270
DYNAMICAL GEOLOGY.
[sec. ii.
boulders from lower to higher levels on the hypothesis of a
gradual submergence of the country while exposed to the
action of coast-ice.' He remarks that fragments of rock
" from being repeatedly caught in the ice and stranded
with violence, and from being every summer exposed to
common littoral action, will generally be much worn ; and
from being driven over rocky shoals, probably often scored.
From the ice not being thick, they will, if not drifted out
to sea, be landed in shallow places, and from the packing
of the ice, be sometimes driven high up on the beach, or
even left perched on ledges of rock."
During such submergence thick deposits of clay and
stones must be formed, and will be subjected in many
places to the friction and pressure of grounding bergs or
of ice-sheets resting on the bottom. We may readily
imagine, therefore, that in these localities compact boulder
clays would be formed, while in other places the action of
currents would sort the materials and arrange them in
stratified beds.
We have actual testimony as to the processes now in
operation on the north-east coast of Labrador, and the
capacity of coast-ice to polish rock-surfaces, and to pro¬
duce boulder clay. Professor H. Toule Hinde ' has de¬
scribed the action of "pan-ice," which is formed by the
breaking up of bay-ice, floes, and coast-ice during the
storms of spring, and consists of large pieces or pans from
6 to 10 or 12 feet in thickness. " This broken ice is pressed
on the coast by winds, and being pushed by the unfailing
Arctic current, which brings down a constant supply of
floe-ice, the pans rise over all the low-lying parts of the
islands, grinding and polishing exposed shores, and re¬
moving with irresistible force every obstacle which opposes
their progress During a period of subsidence the
blocks of strata, boulders, mud, and sand, pushed to and
fro on the shallow sea-bottom by pan-ice, ultimately accu¬
mulate in hollows below its action, and when the debris is
pushed into profound submarine valleys such as exist on
^ "Quart. Journ. Geol. Soc.,''vol. iv. p. 315.
^ "Notes on some Geological Features of the North-Eastern
Coast of Labrador," " Canadian Naturalist," New Series, vol. viii.
p. 236.
CHAP. XIV.J
MARINE DEPOSITS.
271
the Labrador coast, the mass will resemble boulder clay,
and in a sinking marine area it will accumulate to a great
thickness. In a rising area it would be liable to be re¬
modelled by the action of the waves, except in the case of
very deep valleys." '
With regard to the transporting powers of the ice in the
same region. Sir Charles Lyell states, on the authority of
Captain Bayfield, that the coast of Labrador between the
latitudes 60° and 60° N., for a distance of 700 miles, is
strewed over with ice-borne boulders. Some of the blocks
were six feet in diameter, and all of them, both small and
large, were being conveyed in the direction of the prevail¬
ing current, viz., from north to south. Countless blocks
were usually to be seen lying between high and low-water
mark, but sometimes tracts of the coast were observed to-
be bare of boulders, and then again at another season
thickly strewed with these erratics.
In parts of the Baltic stones are frequently frozen
into the ice along the shores ; and, when summer comes,
they are lifted up, and floated away on the ice-rafts.
Dr. Forchhammer relates a remarkable fact which de¬
monstrates how large a number of rock-fragments are
annually transported by ice in the Baltic. A diver went
down to examine a vessel, which had sunk near Copen¬
hagen thirty-seven years before. He found the deck covered
with blocks, from 6 to 8 cubic feet in size, and some of them
pUed one upon the other. He also affirmed that all other
sunken ships in the Sound were covered with similar-
blocks.
Finally, if we consider the extensive areas in the nor¬
thern hemisphere, over which coast-ice is now acting, and
if we regard all the deeply indented coast-lines of Scandi¬
navia, Finland, Greenland, and North America, every yard
of which is more or less subject to this action, we shall
conclude, with Professor Milne, that the coast-ice must, in
quantity, be much greater than that of the glaciers, and
much more effective in the transport of rock débris. " All
the vast ice-fields which break loose from the frozen regions
of the North—and we read of them as 300,000 square miles
in extent, and 7 feet in thickness—are in their passage
south driven in upon the land, and help to grind the coast-
•272
DYNAMICAL GEOLOGY.
[sec. ii.
line, and transport its boulders. The northern field-ice,
■when it arrives in the latitudes of Newfoundland, is often
seen to be covered with boulders, gravel, kelp, and other
materials, sho-wing it to have been, at some time or other,
in contact with the coast." ^ The distance to which these
materials may be transported is indicated by the south¬
ward extension of icebergs in the Atlantic.
' " Geol. Mag.," Dec. 2, vol. ill. p. 408.
CHAPTER XV.
MAEiNE DEPOSITS (continued).
Shallow-Water Calcareous Deposits.
IN this chapter we shall consider all the calcareous
deposits which are being formed in water of less than
300 fathoms in various parts of the world, whether they
are due to direct chemical precipitation, or to the accumu¬
lation of dead shells, corals, and other organic débris.
Both kinds of deposit are really due to the abstraction of
hme from the sea-water by chemical processes, for the
carbonate of hme which constitutes such organic structures
has been obtained by the action of organic secretions on
the salts dissolved in the sea.
I. Deposits formed hy Chemical Precipitation.
It used to be supposed that the free carbonic acid in sea-
water was more than sufficient to keep in solution all the
carbonate of hme usually found in it. More recently, how¬
ever, Herr Tomoe, of the Norwegian North Atlantic Expe¬
dition, asserted that there was no free carbonic acid at all
in sea-water, and that the amounts reported as existing
were really combined with lime in the form of carbonate ;
he appealed also to the fact that sea-water has generally
an alkaline, not an acid, reaction. It seems to be now
admitted that there is more carbonate of lime dissolved in
sea-water than was formerly supposed, but still the quantity
is very small (see Analysis, p. 246) ; and there is also some
free carbonic acid. Dr. Buchanan's observations on board
the "Challenger" show that the free carbonic acid present
is subject to great variations, being greater near the bottom
T
274
DYNAMICAL GEOLOGY.
[sec. ii.
than in surface-water, and also greater in those waters
where animal life is most abundant.
The quantity of dissolved carbonate of lime also varies,
but recent inquiries point to the conclusion that sea-water
is a compound which refuses to hold much carbonate of
lime in solution. Dr. J. Murray has made experiments to
ascertain how much it can take up, and finds that 0*6490
grammes per litre (about five times the ordinary amount
in solution) can be dissolved, forming a clear super¬
saturated solution, but after a time not only the excess so
added is thrown down, but also sometimes a portion of that
normally present in the water.' What, therefore, becomes
of all the carbonate of lime which is being annually carried
into the sea by rivers ? it used to be thought that it was
taken up and used by marine creatures, but it seems more
probable that it is disposed of either by precipitation or by
immediate conversion into sulphate of lime.
Mr. J. G-. Goodchild has suggested that most of the car¬
bonate of lime carried down by rivers is converted into
sulphate of lime. He refers to a statement by Dr. Sterry
Hunt, that if a solution of carbonate of lime in carbonated
water be mixed with a solution of sulphate of magnesia
in water, double decomposition ensues, and carbonate of
magnesia and sulphate of lime are formed. The difficulty
in supposing this reaction to occur lies in accounting for
the carbonate of magnesia that would be formed, for none
exists in sea-water.
It appears to be a fact that where much carbonate of
lime is brought down by rivers, some of this material is
precipitated and forms a calcareous deposit. Thus a hard
calcareous rock is being formed on the sea-bottom opposite
the mouths of the Khone in the Mediterranean, and along
the coast of Languedoc.
Lyell states that large masses of hard rock, consisting of
sand cemented by calcareous matter, are continually found,
and that a cannon, embedded in crystalline, calcareous
rock, taken up from the sea near the river's mouth, may
be seen in the museum of Montpelier.'
Deposits of the same kind are found along the southern
' "Proc. Koy. Soc. Edin.," and " Nature," June 12,1890, p. 165.
^ " Principles of Geology," tenth edition, vol. i. p. 430.
CHAP. XT.J
MARINE DEPOSITS.
275
coast of Asia Minor. Admiral Sir F. Beaufort found that
great alterations had taken place on this coast since the
time of Strabo : havens have been filled up, islands are
joined to the main land, and many miles of new land have
been formed, consisting of compact stone. Almost all the
rivers and streams contain abundance of carbonate of lime
in solution, and the precipitation of this binds together the
sand and gravel in the river deltas, and converts them into
calciferous sandstones and conglomerates.^
In accounting for this deposition of limestone, Lyell
observes " that the fresh water introduced by rivers being
lighter than the water of the sea, floats over the latter, and
remains upon the surface for a considerable distance. Con¬
sequently, it is exposed to as much evaporation as the
waters of a lake ; and the area over which the river-water
is spread, at the junction of great rivers and the sea, may
well be compared, in point of extent, to that of considerable
lakes."
It is probable, also, that a chemical action goes on where
the under surface of this body of fresh water is in contact
with the salt water. Dr. Murray's experiments above
mentioned suggest that such action does take place, and the
precipitation of the carbonate of lime may be due either
to the presence of alkaline salts or to the abstraction of part
of the carbonic acid which kept it in solution. The forma¬
tion of solid limestones in shallow waters by one or both
of these processes is, at any rate, an ascertained fact, so that
the view taken by some writers, that aU marine limestones
have had an organic origin, does not seem to be correct.
The chemical precipitates formed from sea-water by
evaporation and concentration when land-locked lagoons
have been separated from the sea have been described
under the head of lacustrine deposits (see p. 246).
Chemical Reactions of Marine Organisms.—The
capacity of marine creatures to form calcareous shells and
skeletons involves questions of much interest to geologists.
It used to be supposed that they had the power of simply
abstracting the carbonate of lime, and that the quantity of
that mineral found in sea-water was small because most
' Op. dt., p. 431,
276
DYNAMICAL GEOLOGY.
[sec. ii.
of it had been so abstracted ; but if this were so, we should
expect to find that the amount of carbonate of lime in solu¬
tion was still very large near the mouths of rivers, and
that it became less and less toward the open ocean. This,
however, is not the case ; on the contrary, the composition
of sea-water in narrow seas and estuaries is practically the
same as that of water far from land, and yet the amount
of animal life in estuaries is generally less than in the
open sea. Moreover, as we have seen, there are other
ways of accounting for this sudden disappearance of the
carbonate of lime.
How then do marine creatures get the carbonate of lime
for their calcareous structures ? Eecent researches seem
to prove that they have the power of getting it from the
sulphate. Experiments ' by Messrs. Irvine and Woodhead
have shown that animals with a circulatory system can
take up any salts of lime in solution and convert them,
first into phosphates, and finally into carbonates. More
recently Murray and Irvine, experimenting with the liquor
of oysters and mussels, found that it contained a great ex¬
cess of carbonate of lime over that in sea-water, and they
refer this to the direct secretion of carbonate of ammonia
by the cells of the living animals, which, reacting on the
sulphate of lime in the sea-water, is capable of throwing
out nine-tenths of the calcium salt in the condition of
carbonate.' They believe that many marine animals secrete
carbonate of ammonia as an effete product, and referring
to corals, they say ; " In polypes, which, unlike the higher
animals, have no true circulatory system, and where the
animal is immersed in sea-water, it seems hardly possible
to account for the enormous secretion of carbonate of lime
in the manner indicated by Bischoff ; but if the conclusion
we have arrived at be correct, and such animals, in place
of secreting urea, secrete carbonate of ammonia, then we
have a perfectly reasonable explanation of the phenomenon
of coral formation."
It may be added that by this process we are also able to
understand why the sea-water in the neighbourhood of
coral-reefs is sometimes so saturated with carbonate of
' " Proc. Koy. See. Edin.," 1888-9, part xvi.
' " Proc. Koy. Soc. Edin.," and " Nature," June 12, 1890.
CHAP. XV. J
MARINE DEPOSITS.
277
lime that the retiring tide leaves a thin white coating of
it on the pebbles of the beach. Dana describes a beach
on Oahu, one of the Sandwich Islands, where "the pebbles
are covered with a thin incrustation of carbonate of lime,
appearing as if they had been dipped in milk, while others
are actually cemented (together), yet so weakly that the
fingers easily break them apart." On the island of Ascen¬
sion the rocks situated at the end of a long beach of cal¬
careous sand are regularly coated every year with a white,
thick, and hard deposit of carbonate of lime.
Calcareous Mud and Limestone Paste.—As we
shall presently see, the fragments of the calcareous shells,
tests, and skeletons of various kinds of sea creatures, make
up a large part of most marine limestones, sometimes as
much as 90 per cent. ; but besides these definite organic
fragments, there is always a certain amount of fine calca¬
reous sediment which fills up the spaces between the frag¬
ments : and there are some limestones which consist
entirely of such a calcareous paste, with very few recog¬
nizable organic fragments in it.
To account for this paste several explanations have been
suggested : (1) When the chitinous membrane which holds
together a calcareous shell has decayed, the carbonate of
lime is attacked by carbonic acid, and is so loosened by
partial solution that it falls to pieces, and is gradually re¬
duced to a fine kind of powder. (2) Another source of fine
sediment is to be found in the trituration of calcareous
sand in the intestines of sea creatures ; this is an active
process in some localities, especially on coral reefs, where
holothurians, or sea-cucumbers, are very abundant ; th^se
creatures swallow the sand, and obtain nutriment from the
diatoms, foraminifera, and other minute organisms which
occur in it, the hard calcareous particles being voided. But
everywhere all kinds of echinoderms, molluscs, Crustacea,
and fish are actively engaged in a similar process, and must
expel large quantities of triturated calcareous material. Dr.
Guppy states that fifteen or sixteen holothuria are capable ■
of discharging a ton of such material in the course of a
year. This production of calcareous mud is also assisted
by the detritive effect of waves on such portions of calca¬
reous rock as come within their action. (3) There is still
278
dynamical geology.
[sec. ii.
another way in which calcareous mud may be produced,
and that is the action already mentioned of carbonate of
ammonia on sulphate of lime. Messrs. Murray and Irvine
state that if animals such as crabs are allowed to die in a
limited amount of sea-water, the carbonate of ammonia
formed, with other products of decomposition, precipitates
all the calcium sulphate in the water as carbonate. Thus
not only do marine creatures secrete carbonate of lime
while they are alive, but their decay after death also cause a
precipitate of the same substance ; and Mr. Goodchild aptly
remarks, that " such action is not by any means necessarily
limited to the tenants of the sea-floor, for the decomposi¬
tion of pelagic organisms is likely to contribute more or
less to the same result. Nor is it limited to organisms
with calcareous frameworks, but may result just as much
from the decay of, say, jelly fishes or sponges, as from their
lime-secreting allies." '
On these grounds we may safely agree with Mr. Good-
child in concluding that " chemical precipitates due to the
action of decomposing organic matter upon sulphate of
lime have an essential influence upon the formation of
limestones, even when these are mainly of organic origin " ;
and that if these views are correct, we must regard " all
limestones as of compound origin, partly organic, partly
detrital, and partly as chemical precipitates, the proportion
of each to the others varying to a much greater extent
than has yet been recognized."
II. Deposits formed of Organic Debris.
We will now consider the formation and distribution of
calcareous deposits, which consist largely of organic debris.
1. Shell Sands.—The materials for the construction of
calcareous sandstones, or sandy limestones, are being accu¬
mulated round many coasts, and are common in the
English Channel and off the west coasts of France and
Britain.^
In the western part of the Channel deposits of broken
shells and shell-sand form three long, occasionally inter-
' "Geol. Mag.," Dec. 3, 1890, vol. vil. p. 78.
^ See " Lithologie du Fond des Mers," par Delesse, Paris, 1872.
CHAP. XV.J
MARINE DEPOSITS.
279
rupted, bands whicb unite at the mouth of the Channel
between Cornwall and Brittany to form a large spread of
shell-sand. There are also extensive tracts of shell-sand
along the north coast of France, especially along the sweep
of the coast from Calais to Etretat, on each side of the
Cotentin, and in the Bay of St. Brienc. The littoral por¬
tions of these deposits are dug for marling the land.
There is also a large tract of calcareous shelly sand off
the west coast of Brittany, extending seawards for over
60 miles, nearly down to the line of 200 fathoms, where its
front is over 100 miles long.
In the Irish Sea there are frequent records on the Ad¬
miralty charts of shells, or sand with shells, especially in
the central area between Wales and Ireland; there is a
tract of irregular shape and varying width extending from
the coast of Anglesey to the deep water east of Wexford,
a distance of 100 miles, where shells and shell-sand largely
prevail, mixed with some quartz-sand, and occasionally
fine sand or mud.
On our north and north-west coasts beds of shells occur
round the Hebrides and over a bank to the south-west of
the Faroe Islands. Still further west, around the isolated
reef in the Atlantic known as Eockall, there are beds of
shell débris, and also a deposit formed entirely of fish re¬
mains, and extending for a distance of three miles.
2. Limestone Banks.—Immense accumulations of
organic débris are being formed on the sea-floor in some
tropical regions ; one of the best known is that called the
Pourtales plateau, south of Florida. Outside the Florida
reefs, down to about 90 fathoms, the bottom consists of cal¬
careous mud, but beyond that is a wide plateau sloping
gently to 250 or 300 fathoms, and 18 miles wide at its
western end. This wide shelf or plateau has a rough rocky
floor, consisting of the remains of various organisms, deep-
water corals, shells, echinoderms, polyzoa, Crustacea, etc.,
mixed with shell-sand, and frequently cemented together
by carbonate of lime, so that large slabs of solid limestone
were dredged up from it.
Professor A. Agassiz has shown that raised banks of
similar limestone form the foundation on which the coral-
reefs of Florida have been built up; and he describes
280
DYNAMICAL GEOLOGY.
[sec. ii.
other great plateaux or banks in the same seas which con¬
sist of the same materials, especially off the coasts of
Yucatan and Honduras. A large part of Yucatan consists
of precisely similar limestone which has been raised into
dry land, and is now traversed by caverns and fissures,
some of which have been explored to a depth of 400 feet ;
this shows how great a thickness of such material may be
built up by the accumulation of organic débris in seas like
these which are literally teeming with life.
3. Coral-reefs.—Coral-making Actinozoa exist in all
temperate and tropical seas, and are found at various
depths. The great reef-malcing corals, however, cannot
exist in waters where the temperature ever falls below 68°
F. ; they are, therefore, confined to those parts of inter¬
tropical seas which are never invaded by currents of water
colder than 68° F. Moreover, these coral-polyps can only
live in shallow water, and are generally found within a
depth of 25 fathoms, though some species tmder favour¬
able circumstances appear able to grow in 30, or even 40
fathoms. Another condition necessary for the growth of
coral-reefs is clear water. Mud is always detrimental to
animal life, and the coral-polyp is especially sensitive to
the infinx of muddy water brought down from the land.
Even within the tropics the growth of coral-reefs may be
prevented by any of the following canses :—
1. The too great depth of the water.
2. The occasional influx of cold water.
3. The frequent influx of muddy water.
4. The impingement of too strong a current.
Coral-reefs are generally divided into three kinds or
classes,—Fringing-reefs, Barrier-reefs, and Atolls.
Fringing-reefs are those which are formed along the
margin of any coast or island. If the slope of the shore
below water is steep, the reef is a narrow one ; if the slope
is gentle, a wider fringe of reef will be formed, its outward
extension being dependent upon the depth of the water.
It must not be supposed that a fringing-reef always forms
a level platform which is wholly dry at low water ; such
shore-reefs do occur in some places, but the exposed part
of a fringing-reef is generally from half a müe to a mile
from the beach, and sometimes more than a mile. The in-
CHAP. XV.J
MARINE DEPOSITS.
281
termediate space is covered by water, sometimes only 3 or
4 feet in depth, and sometimes 3 or 4 fathoms, so as to
afford a passage for boats. Its floor is generally coral-
sand, with here and there tufts and bosses of living coral.
This channel communicates at intervals with the sea by
openings through the outer reef.
Barrier-reefs are defined by Darwin as differing from the
above " in being placed at a much greater distance from
the land with reference to the probable inclination of their
submarine foundations, and in the presence of a deep-
water lagoon-like space within them." The longest barrier-
reef in the world is that of the north-east coast of Aus¬
tralia ; this extends, with a few interruptions, for nearly
1,000 miles, its average distance from the coast being be¬
tween 20 and 30 miles. The channel or arm of the sea
inside it is from 10 to 20 fathoms deep, with a sandy
bottom, and at its southern end, where the reef is further
from the shore, it is as much as 40 to 50 fathoms deep.
Mr. Jukes ' compares it " to a great submarine wall or
terrace, fronting the whole north-east coast of Australia,
resting at each end on shallow water, but rising from very
great depths about the centre ; its upper surface forming
a plateau covered by 10 to 30 fathoms of water, but
studded all over with steep-sided block-like masses which
rise up to low-water level. These masses are especially
numerous and most linear along the edge of the great bank
on which they rest, the passages between them being often
very narrow, like regular embrasures opened here and there
through the parapet wall of a fortress. These ' individual
reefs ' running along the outer edge protect the compara¬
tively shallow water inside, and with the numerous inner
reefs that are scattered over its space make it one great
natural harbour."
New Caledonia, in the western Pacific, has a barrier reef
off its western coast which is 400 miles long, and seldom
approaches to within 8 miles of the shore.
Some islands are entirely surrounded by a reef which
lies at a distance of several miles from the shore, and is
separated from it by a deep-water channel ; such reefs
' " Manual ofGeology,"second edition,p. 131. See also " Voyage
of H.M.S. ' Fly,' " vol. i. ch. xiii.
282
DYNAMICAL GEOLOGY.
[sec. ii.
were termed by Darwin " encircling barrier-reefs." Vani-
koro, in the South Pacific, has such an encircling reef, and
the inside channel is in some places 40 to 50 fathoms deep.
Some encircling reefs have only a very small island in
the middle of a large lagoon-like space of water. Between
such reefs and atolls there is very little difference, an atoll
being a reef that encircles a lagoon without any island,
except such as consist of coral.
Atolls are of all sizes, and almost all shapes, sometimes
nearly circular, sometimes long and narrow, and varying
from half a mile to 50 or 60 miles across. The lagoon may
be shallow, but is generally deep. The reef generally sup¬
ports a string of islands which are entirely composed of
coral-rock, the part above the sea-level consisting of
broken corals and coral-sand drifted up by the waves and
winds, and subsequently consolidated into rock. The
living reef outside is submerged at high water, but at low
tide a broad, bare, rocky plateau is presented to view with
pools and holes, which swarm with molluscs and other
animals ; this plateau being often intersected and broken
up by winding creeks and channels.
The view of a coral-reef has been thus described by Mr.
Jukes ; ^—" A coral-reef is a very different, and in itself a
much less interesting and beautiful object than I had ex¬
pected ; neither are the colours of the living corals so
splendid as is often said. The mass of the reef is a dark-
brown, close, hard rock, with here a patch of white sand,
and there a lump or a mass of living coi-als, some of the
smaller of which are grass-green, yellow, or red. When
sailing among the islands, however, the reef and the
islands often formed most beautiful objects, as seen from
the poops, or mizen-top. The little island, crowned with
trees, is surrounded by a smooth space of clear, shallow
water of a bright, grass-green colour, round which is a
narrow fringed margin of white foaming surf, sparkling
in the sun, dividing the lagoon from the rich clear blue of
the deep water outside."
The seaward or windward side of coral-reefs is always
exposed to the battering and pounding action of a heavy
^ " Letters of J. B. Jukes," Chapman and Hall, 1871, p. 173.
Fig. 67. View of a Coral Island (Atoll).
284
DYNAMICAL GEOLOGY.
[sec. ii.
surf, since the long roll of the ocean-swell falls suddenly
upon the upper edge of the reef and dashes over its sur¬
face in huge breakers, that are sometimes felt even all
across it. If a mass of coral, living or dead, be once de¬
tached from the rest, it is soon acted upon by these
breakers, and ultimately triturated into calcareous sand.
The large solid corals, such as Porites, Astraea, and
Mseandrina, grow chiefly along the outer margin of the
reef where the breakers are continually rolling over them.
The more delicate and branching forms, like Explanaria
and Madrepora, flourish in the central lagoon, or in the
more sheltered channels between the outer reefs. The top
of the reef is often encrusted and heightened by a growth
of calcareous seaweed called Nullipora, which sometimes
forms a stony coating two or three feet in thickness, and
resists the force of the waves.
Construction of Coral-rock.—Jukes describes the compo¬
sition of the solid reef-rock in the following terms : '—
" Coral-reefs consist of living corals only in parts of their
upper surface, and along their outside rim, which is a mere
film compared with their whole bulk. All the interior is
composed of dead corals and shells, either whole or in
fragments, and the calcareous portions of other marine
animals. The interstices of the mass are filled up and
compacted together by calcareous sand and mud, derived
from the waste and debris of the corals and shells and of
countless myriads of minute organisms, mostly calcareous
also. The living part, even of the upper surface of the
reef, is that only which is never dry at low water. The
part which is then exposed is composed of mere stone,
which is often capable of being split up and bfted in slabs,
bearing no small resemblance to some of our oldest lime¬
stones. These slabs and blocks, when broken open, are
frequently found to have a crystalline structure internally,
by which the forms and the organic structui'e of the corals
and shells are more or less disguised and obliterated. A
coral-reef, then, of which a part is still living and in process
of formation above, may internally consist of solid crystal¬
line limestone."
^ " Manual of Geology," third edition, p. 388.
CHAP. XV.]
MARINE DEPOSITS.
285
Upon this compact basis the wind often raises banks of
calcareous sand derived from the waste of corals and
shells, and these are compacted into masses which are very
different from the lower portion of the reef. "The large
fragments of corals and shells are never found much
beyond the surf range of high-tide, and therefore always
form rock at a low level ; whilst, on the contrary, the fine
calcareous sand is removed by the wind and deposited in
irregularly laminated beds, which being consolidated in
various degrees, are converted into rock of different
qualities." ' This consolidation is effected by the action of
percolating rain-water, which dissolves some of the car¬
bonate of lime from the surface layers and re-deposits it
below in the form of a calcareous cement, so that the loose
grains of sand are thus bound together into a solid mass.
Captain Nelson describes the ordinary coral rock of
Bahama as forming a strong and homogeneous calcareous
sandstone, soft and loose near the surface, but harder
below, and capable of being used for building purposes.
" It is somewhat similar to Portland stone in appearance,
but softer and more porous. When first exposed it is
quite white, but becomes of a dark ashen-grey colour
along the sea-coast, and elsewhere when exposed to the
weather. On some reefs the rock formed in this manner
is distinctly oolitic, each particle being enveloped in two or
three concentric coats, like the coats of an onion."
Among coral-rocks the following varieties may be dis¬
tinguished :—
1. A rock consisting of corals standing as they grew,
the interstices being filled with broken fragments, shells,
and coral-sand.
2. A breccia of large blocks of coral and previously
formed coral-rock embedded in sand, and often cemented
together by carbonate of lime.
3. A rubbly rock consisting of broken pieces of coral,
with shells and other organic debris.
4. A fine-grained rock consisting of small fragments of
corals, shells, echinodeims and nullipores, with foramini-
fera, which are sometimes very abundant.
^ Nelson on the Bahamas, " Quart. Journ. Geol. Soc.,'' vol. ix.
p. 206.
286
DYNAMICAL GEOLOGY.
[sec. II.
5. Fine-grained compact limestones formed by the ce¬
mentation of the rocks grouped under No. 4.
6. Oolites formed of round concretionary grains.
The first three are reef-rocks, the last three are lagoon
and beach-rocks.
Coral Mud.—Fine calcareous mud is also formed by
the further trituration of the dead corals and coral-sand,
and is often accumulated in the quiet waters of the in¬
ternal lagoons and channels. Such is the case in some
of the inner channels and lagoons of the Bahama Islands,
especially along the western coast of Andros Island, which
is bordered by an extensive deposit of fine, chalky mud,
thus described by Captain Nelson : '—" The western shore
of the island is generally a stiff chalk-bank, four or five
feet high, sloping inwards to the marshy chalk-flat which
surrounds the fresh-water lake of Andros Island ; at its
eastern limit it abuts agaiust a narrow ridge of hiUs along
the centre of the islands. Parallel to the western coast
and seaward, with a breadth of 14 or 15 miles, the chalk
continues in mass as an anchorage," and beyond this it is
spread out as a thin covering over the sandy bottom for a
space of about 80 miles in length by about 60 in extreme
width, the area being rudely heart-shaped.
Where slopes are steep, as they are near barrier-reefs
and atolls, some of the mud so formed is carried out by
the tides into the deeper water outside the reefs, and is
spread over the bottom below the 50-fathom line, where it
forms a thick deposit of fine calcareous mud, or ooze.
Mr. Jukes states that wherever deep soundings were taken
round the north coast of Australia, the " bottom " was
found to consist of a very fine, almost impalpable, pale
olive-green mud, which was entirely soluble in hydro¬
chloric acid, and therefore consisted of carbonate of lime.
The greater part of this material is probably obtained
from the washings of the coral reefs, though a certain
proportion may be derived from the shells of oceanic
moUusca.
The soundings obtained by the " Challenger " expedition
near the Bermudas, from various depths within a certain
• "Quart. Journ. Geol. Soc.," vol. ix. p. 208.
CHAP. XV.]
MARINE DEPOSITS.
287
distance of the reefs, showed that in all cases the bottom
consisted of " soft, white, calcareous mud, evidently pro¬
duced by the disintegration of the Bermuda reef and of
the multitude of Pteropod shells which sink down from
the surface." ^
In the contracting lagoons of certain coral islands, it
appears possible for a compound of the carbonates of lime
and magnesia (called Dolomite) to be formed. Analyses
of the coral limestone of Matea have demonstrated that
magnesia is largely present in some specimens of the rock,
one specimen affording as much as 38 per cent. Dana
thinks that this limestone must have been formed in a
lagoon where the soft coral mud was mixed with a deposit
of maguesian salts obtained from the sea-water as the
solution became gradually concentrated by evaporation.
Thickness of Beef-rock.—As reef-building corals can
only live in water of a certain depth, it might be supposed
that coral limestones would seldom be of any great thick¬
ness ; that in a stationary area they could not be much
more than 200 feet thick, that in a rising area the thick¬
ness would be less, and could only greatly exceed that
amount in an area which had for a long time been slowly
subsiding beneath the sea.
It must be remembered, however, that the outside of
every reef is constantly exposed to the action of the waves,
and that much of the dead coral is broken up into sand,
which often accumulates on the slopes outside the reef,
and is arranged by currents in beds and banks of coral-
sand. Where the outside slopes are gentle, portions of the
sea-floor are being continually built up to within 80
fathoms, and may then be occupied by coral-polypes, and
be converted either into extensions of the shore-reef, or
independent and isolated outer reefs. The precise mode
of growth must depend partly on the conformation of the
sea-floor and partly on the strength of the currents;
where the sand is kept in constant motion corals wUl not
grow, but where it is at rest they find a footing.
There is therefore no reason why several hundred feet
of calcareous detritus should not he deposited outside a
1 " Voyage of the ' Challenger,' " vol. i. p. 289.
288
DYNAMICAL GEOLOGY.
[sec. ii.
reef, or why the reef itself should not in time extend over
a large portion of it, the entire mass being eventually
consolidated into a limestone, the whole of which would, if
elevated, be called " coral-rock."
As a matter of fact the thickness of " coral-rock " on
some raised coral-islands is known to be more than 200
feet. Thus in Barbados numerous deep wells have been
made which prove the rock to be in some places 230 feet,
and in one case 260 feet, but observation and inquiry
showed that the lower part of the raised reefs generally
consisted of débris-rock, without corals in position of
growth, and varying in thickness from 5 to 60 feet.'
The greatest actual proved thickness of any one mass
of coral-rock is in Oahu (Hawaian Islands), where a boring
is said to have been carried for 900 feet through " hard
ringing coral-rock into sand and lava," but how much of
this contained coral-blocks is not known. Another boring
was 1,500 feet deep, passing through beds of gravel, coral,
clay, and lava, one mass of coral-rock being 505 feet thick,
resting on 195 feet of clay and gravel, beneath which was
again 28 feet of coral-rock. Such a superposition suggests
subsidence, but the latest movement of the island has
certainly been one of upheaval,® and there is no kind of
barrier-reef around it.
In an area of subsidence it is obvious that a great
thickness of actual reef-rock might be formed, because the
corals could grow upwards as the land sank, and doubtless
would do so in preference to growing outwards. Here,
however, we are met by a difficulty. It is always difficult
to prove subsidence (see p. 65) : how then are we to
know that an island of which no historical records exist
has sunk, and how estimate the extent of subsidence ?
Deep borings are few, and have not been watched by
scientific men. It has been supposed, however, that
barrier-reefs and atolls are in themselves proofs of subsi¬
dence ; this is Darwin's theory of the origin of such reefs,
and this theory we must now state and examine.
Theories of Coral Growth.—Darwin pointed out what
' See "Quart. Journ. Geol. Soc.," vol. xlvii. pp. 211, 216.
^ See Professor A. Agassiz, "BuU. Mus. Comp. Zool.," 1889,
p. 152.
chap. xv.]
MAEINE DEPOSITS,
289
has been already stated, that there is the closest connec¬
tion between atolls and encircling barrier-reefs, and that
one might be formed on the basis of the other if the island
foundation had gradually sunk. In the same way he said
a fringing-reef might by subsidence be converted into a
barrier-reef.
The upward growth of a coral-reef over a sinking island
is illustrated in fig. 63, the horizontal lines, s\ s^, s^ repre¬
senting the sea-level at different periods, and indicating
the amount of successive subsidences. When the sea was
at the level s', the mass of the island was above water and
the fringing-reefs, f f, were formed round its shores. Con¬
tinual, but gradual, subsidence carried the land downward
while the reefs have grown nearly vertically upward, imtil
the sea-level came to coincide with the line s'^. By this
Fig. 68. Conversion of a Fringing Reef into an Atoll
by Subsidence.
submergence the limits of the land would be much con¬
tracted, and the distance between its shores and the
encircling reefs would be correspondingly increased, so that
the latter were eventually converted into barrier-reefs, b, b.
At the stage s', these reefs lay at a considerable distance
from the coast of the sinking island, and were separated
from it by a wide lagoon.
If the subsidence still continued, the width of this lagoon
would still further increase, and every part of the land
would be covered with a growth of coral, until the whole
was converted into an atoll, a, a, or ring of reefs without
a central island ; except, perhaps, an inner reef raised upon
the last disappearing point of land, as at i.
This theory, at once so simple and comprehensive, met
with universal acceptance, and was for many years be-
4
T
A
290
dynamical geology.
[sec. ii.
lieved to be the true explanation of the origin of such
reefs. It is, indeed, very probable that some reefs and
atolls have been formed in this way, the mistake made by
Darwin, and still more unreservedly by Dana, was in
assuming that all barrier-reefs and atolls had been so
formed, and that every such reef might be considered as
a proof of subsidence.
More recently Professor Semper, from a study of the
Pelew Islands (1863) ; L. Agassiz and Leconte, of the
Florida Reefs (1861-66) ; Dr. J. Murray, from the " Chal¬
lenger " Expedition (1880) ; A. Agassiz, writing on the
Florida Reefs (1882) ; Dr. Guppy, on the Solomon Islands
(1884-87) ; and Mr. J. C. Bourne, on the Coral Formations
c c
a a. The original bank, b b. Deposit of organic débris, c c. Coral.
s s. Level of the sea.
of the Indian Ocean (1888), have all dissented from
Darwin's conclusions. These writers have pointed out
the following facts among others :—
1. The three kinds of reefs,—Fringes, Barriers, and
Atolls,—frequently occur in the same group of islands.
2. That all these kinds also occur within regions which
are known to have been recently elevated, as, for instance,
the Florida region and that of the Solomon Islands.
3. That recent observations have shown how submarine
banks and ridges may be built up by accretion till they
form foundations for coral-reefs.
The careful examination of the reefs of Florida and the
adjacent islands, by Professor A. Agassiz, leaves little
doubt that they are based upon the great sheU-banks
which exist in those seas, and which are still being formed
(see p. 279).
CHAP. XV. J
MARINE DEPOSITS.
291
Fig. 69 shows how a suhmarine bank, ridge, or eminence
may be heightened by the gradual accumulation of organic
debris till its surface is brought within the depth of water
in which reef-corals can live, and how an atoU-like reef may
then be built upon it.
Ocean shoals are much more numerous than was sus¬
pected in Darwin's time, and every submarine eminence
is being heightened by the accumulation of organic debris
on its surface. Mr. Murray has estimated that in tropical
uo
fSS- . '5-i
M '¡tí -,
Sù/nd 44- Vi) ;
•' /ftrl:
il to
' )sr.
40
if
■■ i4.-
'21
CtfioZ
4?
.'¿tí}
Vífí'
smZ
B-Oj *S
Soma
44*
\ 4^ "2,9.,
\ g SOAli
« M-
30
c&
31,-—
SB ...
4%y
-'H'îb
/
/
r»
JoJ
+1 3s, ;
^ ^ -SO/ujÍ /
io /
l-'
X-''
Fig. 70. Chart of Macclesfield Bank (after Gapt. Wharton).
Scale 20 sea-miles to an inch. Soundings in fathoms.
seas a square mile by 100 fathoms deep contains over six¬
teen tons of carbonate of lime in the shape of living
organisms ; and besides this there are all the creatures
which grow or creep on the surface of the shoal itself.
Again, in volcanic regions cinder-cones often form tem¬
porary islands, which are gradually worn down by the waves
till only a submarine shoal rerhains, and this would form
a suitable foundation for a coral island.
Finally, Captain Wharton has called attention to certain
292
dynamical geology.
[sec. ii.
submarine banks which present the appearance of being
atolls in the process of formation.' Thus the Macclesfield
Bank in the China Sea is a large oval area 70 miles in length,
the central part of which is covered by 40 or 50 fathoms
of water, but all round its rim are a series of growing
coral-reefs, some of which rise to within 10 fathoms of
the surface (see fig. 70). The Tizard Bank in the same
sea presents similar features, but in this case some of the
reefs have actually reached the surface, while others are
still 4 or 5 fathoms deep.
Dr. Coppinger, describing the Admirante Islands in the
Indian Ocean,' says they stand on an oval-shaped bank
which has a raised rim and a central depressed area, and
he remarks that several of the islands present positive
evidence of elevation. The African Islands exhibit similar
features. Here, therefore, we have banks with the aspect
of submerged or imperfect atolls which bear direct evidence
of upheaval, not subsidence.
The Tonga Islands in the Pacific afford instances of
raised coral-limestones resting on stratified volcanic ash
and tuff. Falcon Island, which is one of the group, and
was described on p. 45, shows how this basis may be pre¬
pared ; and Mr. J. J. Lister has shown that elevation has
been in progress for a great length of time, raising coral-
reefs on Eua and Vavau to heights of 500 and 1,000 feet
above the sea.'
Given a foundation, we only need an explanation of the
special coral growth on its border to understand the
formation of an atoll. This is not difficult. All writers
(Darwin included) agree that the corals which establish
themselves on the outer parts of a shoal are likely to grow
more vigorously than those on the inner parts, because they
get a more abundant supply of food in the sweep of the
ocean currents. The currents themselves too exert a
directing influence : several observers, notably Semper,
Bourne, and Hickson, lay stress on this influence, and
state that corals wiU not flourish in a very strong current
(6 or 7 miles an hour), nor in still water, but grow
' " Nature," Feb. 23, 1888, p. 393.
' " Cruise of the * Alert,' " p. 225.
' " Quart. Journ. Geol. See.," vol. xlvii. p. 590.
CHAP. XY.J
MARINE DEPOSITS.
293
luxuriantly in places where a moderate current flows over
them. Such conditions exist on the outer slopes of a sub¬
marine bank more than over its central part.
Moreover, when once the outer reefs have grown up so
far as to come within the influence of wave action, other
causes come into play ; broken fragments and dead coral-
blocks are piled on these reefs, while fine sand and sediment
are carried inward over the lagoon area, and are kept in
movement by every tide. This exerts a great repressive
influence on the coral growths of the central space ; and this
repression is aided by the boring and' excavating action of
certain worms. Molluscs and Echini, which attack dead
coral, and possibly also (as Murray suggests) by the actual
solution of some of the dead coral. S^ediment in motion,
however, seems to be the chief check to coral growth, and
where the scour of the tide is not very great the lagoon is
sometimes being choked up with sediment, as in the Cocos-
Keeling Atoll.
Putting all the preceding considerations together, we
arrive at the conclusion that Darwin's theory cannot be
accepted as a general explanation of Atolls and Barrier-
reefs, and that the inferences drawn from it as to the great
thickness of coral-rock beneath some reefs cannot be enter¬
tained without further evidence.
Both Professor Agassiz and Dr. Guppy truly remark
that Darwin and Dana were not justified in assuming the
submarine slope outside a coast-line to be a prolongation of
the exposed land-slope. Such a mode of reasoning is very
fallacious, and cannot lead to accurate results. Experience
has shown that submarine slopes are very irregular, and are
often broken into a series of gentle inclines with short,
steep slopes between them. It has also been found that
there are such things as submarine barrier-reefs as well as
submarine atolls, that is to say, wall-like reefs which rise
from ledges where the corals are forced to grow upward,
being limited on the one side by deep water and on the other
by unstable sand or mud derived from a reef nearer the
shore which is exposed to wave-erosion.^
• See Guppy, " Proc. Koy. See. Edin.," 1886, p. 880; Jukes-
Browne and Harrison, " Quart. Journ. Geol. Soc.," vol. xlvii.
p. 204 ; and Captain 'Wharton, " Nature," 1888, p. 395.
CHAPTEE XVI.
DEEP-SEA AND OCEANIC DEPOSITS.
WE have now to consider the deposits which are being
formed in the great oceanic depressions which lie
outside the shelf or zone of comparatively shallow water
which border the continental areas. These deposits may be
classified as follows ;—
1. Blue muds.
2. Grlauconitic marls and sands.
3. Calcareous oozes.
4. Ked clays.
5. Siliceous oozes.
1. Blue Muds.—It was mentioned on p. 267 that the finest
part of the sediment derived from the waste of land-sur¬
faces is carried for a certain distance into the seas and
oceans. This sediment forms a layer of blue mud, and
there is generally a band of such mud immediately outside
the sands which border the coasts of the great continents.
It often extends to considerable depths, but seldom covers
a large area except where great rivers discharge into wide
bays or gulfs. Thus in the Arabian Sea and in the Bay of
Bengal blue mud extends to a distance of 700 miles from
land and down to depths of more than 2,000 fathoms, and
covers an area of about 1,700,000 square miles.^ Dr.
Murray describes these muds as soft and reddish at the
surface, but stiff, blue, and tenacious underneath. They
contain much more organic matter than the more remote
oceanic deposits, and it is the slow decomposition of this
which, by reducing the higher oxides, gives the mass a
* See map and description of the deposits in the Indian Ocean,
by Dr. J. Murray, in the " Scottish Geographical Magazine,"
August, 1889.
CHAP. XVI.J DEEP-SEA AND OCEANIC DEPOSITS.
295
bluish tint. The mineral particles which make up the
bulk of the deposit are of various kinds, and quartz grains
are generally present.
Along the east coast of South America there is a deposit
of analogous origin and composition, but it is red instead of
blue, deriving its colour from the presenceof peroxide of iron.
Green Marls and Sands.—These occur chiefly along high
and bold coasts where no great rivers enter the sea, and
where consequently deposits accumulate much more slowly
than in the regions where blue mud occurs. The material
of the green deposits seems to be partly terrigenous and
partly pelagic, but the mineral particles appear to have
been long exposed to the action of the sea-water, and to have
undergone much alteration. Much of the green matter in
the marls seems to be of organic origin, for when heated on
platinum foil it burns, leaving a residue of carbon and
red oxide of iron, but there are also grains of a green
mineral known as glauconite, which is a silicate of alumina
and iron. Shells of Poraminifera and Oceanic Mollusca
are abimdant, and often form over 60 per cent, of the mass.
A great number of these shells are filled with glauconite,
either dark green, pale green, or yellow in colour, and where
the finer sediment is washed out by a current the remainder
forms a green sand. Such green sand occurs on the
Agulhas Bank south of Africa, and below the Gulf Stream
between Florida and Cape Hatteras,'in depths of from 100
to 1,000 fathoms. Glauconite concretions containing
phosphate of lime also occur at both places.
Volcanic islands are always bordered by deposits of sandy
marl or mud, consisting of mineral particles derived from
volcanic rocks, with varying proportions of Poraminifera and
Oceanic Mollusca, but seldom, if ever, any glauconite. The
coral muds which similarly surround coral islands have
already been described.
Pteropod Ooze.—The surface waters of the warmer
regions of the great oceans swarm with small Molluscs
belonging to the classes Pteropoda, Heteropoda, and the
young fty of Gasteropoda. All these have exceedingly
thin and delicate shells, and, like all the minuter forms of
animal hfe, they make up for their individual insignificance
by their immense abundance, so that they actually supply
296
DYNAMICAL GEOLOGY.
[sec. ii.
food for so large a creature as the whale. Their dead
shells, however, are not found everywhere on the ocean-
floors as might have been expected, but only where depths
of from 200 to 1,000 fathoms occur at some distance from
continental land ; their absence at greater depths being due
to the solvent action of the sea-water, in which they are dis¬
solved before reaching the bottom.
Pteropodoozes contain from 70 to 90 per cent, of carbonate
of lime, made up chiefly of the shells of pelagic Mollusca, but
including many other organisms which live on the bottom,
such as deep-water corals, echinoderms, alcyonaria, and
4. 5
Fig. 71. Foraminifera, highly magnified.
1. Glohigerina. 2. Textularia. 3. A'erneuilina. 4. Cristellaria.
5. Kosalina.
foraminifera. They frequently form a zone or band of de¬
posit between the muds above-mentioned and the ooze which
is described below.
Glohigerina Ooze.—Foraminifera are minute animals be¬
longing to the sub-kingdom Protozoa, and they exist in
countless myriads in all warm and temperate seas, the cal¬
careous shells which they produce being of the most varied
and beautiful forms. They generally consists of several
chambers, each of which is perforated by numerous holes,
through which the delicate tentacular processes of the
animal are protruded.
CHAP. XVI.J DEEP-SEA AND OCEANIC DEPOSITS. 297
The deep-sea soundings carried out across the North
Atlantic by Captain Dayman (1857), and more recently by
the officers of the " Challenger " expedition (1873-6), have
made us well acquainted with the bed of that ocean. At
a distance of about 200 miles from the west coast of Ireland
the bed of the Atlantic slopes down rapidly to a depth of
1,700 fathoms, and thence there is a great undulatiqg sub¬
marine plain which extends to within 500 miles of New¬
foundland, when the bottom slopes gradually up again till
it rises into dry land. This great plain has been called the
telegraphic plateau ; from east to west it measures about
1,300 miles, and from G-reenland on the north to the Azores
on the south its extent is about the same, most of this
space being about 2,000 fathoms deep, and no part of it
being deeper than 2,700 fathoms, or 16,200 feet.
Specimens of the mud at the bottom were brought up by
the sounding machines during the various surveys, and
that which is found all over the central space between
Ireland and Newfoundland, and also far to the southward,
is a kind of soft sticky or mealy substance, which was de¬
scribed by Captain Dayman under the name of ooze. This
ooze is of a whitish, or greyish-white tint, and when dry re¬
sembles very fine chalk. When examined under the micro¬
scope it is found to consist chiefly of the shells of Foramini-
fera, and as more than three-fourths of these belong to the
one genus Gldbigerina, it has therefore been termed the
Globigerina ooze. When treated with a dilute acid, a violent
effervescence takes place, and the greater part of its bulk
disappears, being cbiefly carbonate of lime. The insoluble
remainder, usually about one-tenth by weight of the whole
mass, consists partly of siliceous matter of organic origin,
and partly of minute particles of pumice, felspar, augite, and
other minerals which occur in volcanic rocks. The trans¬
port of volcanic dust to great distances by the winds, and of
pumice by marine currents, readily accounts for the occur¬
rence of these mineral fragments.
The analyses of calcareous oozes vary considerably, but
the following is the composition of three samples : ' two are
Globigerina oozes from the North Atlantic, No. I from the
' " Voyage of the ' Challenger,' " vol. ii. pp. 374, 380.
298
DYNAMICAL GEOLOGY.
[sec. ii.
depth of 1,900 fathoms, and the second from a depth of
1,420 fathoms ; the third is a Pteropod ooze from 450
fathoms off Sombrero Island in the West Indies :—
No. 1.
No. 2.
No. 3.
Loss on ignition . . .
6-63
3-80
4-00
Alumina and ferric oxide
5-86
4-42
4-80
Calciupi sulphate . . .
0-51
2-82
1-00
Calcium carbonate. . .
74-50
80-69
84-27
Magnesium carbonate. .
1-27
0-68
1-28
Soluble silica ....
5-80
4-14
2-60
Residue, chiefly silicates .
5-63
3-45
2-05
100-00
100-00
100-00
It was for some time a question whether these G-lobige-
rinae lived at the bottom of the ocean where their shells
are now found, or whether they lived in the surface waters,
their shells only falling to the bottom when the animals
died. The observations of Dr. Murray, during the voyage
of the " Challenger," proved the latter to be the truth.
He found that the Globigerinse swarmed at and near the
surface in such numbers that their shells must fall hke an
incessant rain through the waters of the ocean, and must
be continually adding to the deposit at the bottom. Pro¬
fessor Huxley has calculated that if we assume this fora-
miniferal shower wiU form in one year a layer of solid
matter one-tenth of an inch thick, "then if the present
state of the Atlantic Ocean has existed for only 100,000
years, this apparently unimportant operation will have
sufficed to cover its floor with the materials of a bed of
limestone no less than 800 feet thick." ^ Looking only to
the shallower parts of the Atlantic, where the deposits
bear a greater resemblance to chalk than those of the
deeper waters, we see that if these were gradually uplifted
above its surface, the land so produced would consist of a
^ "Physiography," p. 267.
CHAP. XVI.J DKEP-SEA AND OCEANIC DEPOSITS.
299
chalky material similar to that which occupies so large a
portion of the continent of Europe.
The Glohigerina ooze of the North Atlantic has been
specially described because the first explorations were made
in that ocean, but it is also the prevalent deposit in the
South Atlantic, Pacific, and Indian Oceans, wherever the
depth does not exceed 2,400 fathoms. In depths of less
than 2,000 fathoms the ooze contains from 50 to 86 per cent,
of carbonate of lime ; but below this depth the amount
rapidly decreases, the shells in it having a more and more
rotten and decayed appearance ; at the same time the ooze
assumes a grey, and finally a reddish or brownish tint, and
passes either into a red clay or a Eadiolarian ooze (to be
described hereafter). It would appear, therefore, that the
falling shells suffer from the dissolving action of the car¬
bonic acid in the sea-water, and that where they have to
fall through more than three miles of water they lose a
large portion of their substance, and are finally altogether
dissolved. It would also seem that this dissolution pro¬
ceeds more rapidly at great depths, probably because of
the great pressure at such depths.
Red Clay.—The central part of the Atlantic Ocean is
divided into two great basins by a sub-oceanic ridge, now
known as the Dolphin Bank. The Azores are situate on
this ridge, which slopes on each side down to depths of over
2,600 fathoms, and in two places to over 3,000 fathoms.
The South Atlantic is similarly divided by the Challenger
Bank, and has similar depths. The Pacific has large areas
which are over 8,000 fathoms, and one part which is over
4,000. The central part of the Indian Ocean is between
2,000 and 3,000 fathoms, only three small areas being over
the latter depth.
It has been stated that at depths of about 2,400 fathoms
the ooze assumes a reddish tint ; this is due to the higher
oxides of iron and manganese, and where süiceous or¬
ganisms are few, the G-lobigerina ooze passes into a red
or reddish-brown clay, which, in the greater depths, con¬
tains merely a trace of carbonate of lime. The passage
from ooze to red clay is often gradual, so that some sam¬
ples which are classed as red clay contain many Foramini-
fera and yield from 20 to 30 per cent, of carbonate of lime.
300
DYNAMICAL GEOLOGY.
[sec. ii.
In greater depths, however, the bulk of the deposit (over
80 per cent.) consists of fine amorphous clayey matter
mixed with very minute mineral particles ; in this are
scattered a few larger particles of volcanic minerals, and
often a large quantity of disintegrated fragments of pumice.
Peroxide of manganese is also abundant, both in the form
of small grains and of large nodules or concretions.
Dr. J. Murray's view of the origin of this remarkable
deposit is now generally accepted ; he points out that the
materials of which it is formed are present in the Grlobi-
gerina ooze, but are there masked by the large amount of
calcareous material ; if, however, this material is removed,
the residue is similar in composition to the red clay, only
differing in having a larger proportion of definite mineral
fragments. He naturally infers that the red clay and the
ooze residue have the same origin, and that they are
entirely derived from the fine dust which is carried by the
winds, and from the pumice which floats on the surface of
the ocean. Both the pumice and much of the dust are
volcanic ejections (see p. 28) ; the universal distribution
of pumice over the ocean and the ocean-floor, and on the
shores of oceanic islands, was established by the experiences
of the " Challenger " expedition. The dredge often brought
up bushels of waterwom pieces of it, from small grains to
large lumps. Besides this volcanic material, fine terrestrial
dust is often borne to great distances, and may contribute
to the formation of the red clay. ,
When the particles of dust reach the bottom many
undergo slow decomposition, and ultimately furnish the
fine amorphous material which forms so large a part of the
deposit. If this view of its formation be true, its accumu¬
lation must be a very slow process, and of this there are
several proofs. The teeth of sharks and the ear-bones of
whales are much more common in the red clay areas than
any where else, but as there is no reason to suppose sharks
and whales are any commoner in the waters above them, it
can only be that they remain unburied for a much longer
time than where ooze is being formed. Again, some of the
teeth dredged up belong to extinct species, a striking proof
of the slowness of accumulation in these depths. Lastly,
in the red clay there is a much larger proportion of par-
CHAP. XVI.J DEEP-SEA AND OCEANIC DEPOSITS.
301
tides of meteoric iron, and as this is of universal distri¬
bution as cosmic dust, its relative abundance can only be
accounted for in the same way.
Radiolarian Ooze.—The Eadiolaria inhabit all the
warmer seas and oceans of the globe, and sometimes
swarm in such numbers as to discolour the surface of the
water. They can live at all depths, and different species
are found to inhabit different zones of depth in the ocean,
and certain forms have only been obtained from the bottom.
1 2 3
Fig. 72. Kadiolaria, very greatly magnified.
I. Podocyrtis. 2. Stylodictya. 3. Encyrtidium.
Those most commonly met with belong to one of two
groups, the Polycistina and the Acanthometrina. The
former construct a delicate external shell like that of Fora-
minifera, only siliceous instead of calcareous ; the latter
form an internal framework or skeleton of siliceous spicules
radiating from a central point.
The tests of Polycistinse occur in most deep-sea deposits,
and were especially abundant in the deepest soundings
examined by the staff of the " Challenger " expedition.
Thus, from the depth of 4,575 fathoms in the Pacific, a
fine gritty reddish deposit was obtained, which proved to
302
dynamical geology.
[sec. ii.
be so largely composed of Kadiolaria that Dr. Murray pro¬
posed for it the name of Radiolarian Ooze. There is, in
fact, no difference between red clay and some of the Eadio-
larian ooze, except the jiresence of Eadiolaria ; when these
make up 25 per cent, of the mass it is called Eadiolarian
ooze. The sihceous organisms often make up 60 or 70 per
cent, of the ooze, and include Diatoms and fragments of
the spicules and skeletons of siliceous sponges.
From his Pacific experiences Sir Wyville Thomson
thought that this ooze belonged to still greater depths than
the red clay, the latter passing down into it by an increase
in the number of siliceous organisms ; he says ; " If the
Eadiolarians live at all depths in the sea, the number of
their skeletons falling to the bottom at one place must
increase with the increasing depth of the water, and it
becomes quite intelligible that, in a bed which is being
formed at the prodigious depth of five and a haK nautical
miles, the tests of the Eadiolarians should so preponderate
■over the red clay as to entirely alter the character of the
deposit." ' Further exploration, however, has not quite
•confirmed this view ; there is often a passage from red clay
to Eadiolarian ooze, but it is a lateral one and not depen¬
dent on any increase of depth. Moreover Globigerina ooze
sometimes passes directly into Eadiolarian ooze without the
intervention of any red clay.
The abundance of the shells of Eadiolaria on some parts
of the ocean-floor, and their rarity on other parts, has not
yet been fully explained, but Dr. Murray observes that they
appear to be more abundant in those parts of the ocean
which have a relatively low degree of salinity, and they are
not abxmdant in the Atlantic where the salinity is generally
high.
In Barbados there is an interesting series of raised
oceanic deposits, consisting chiefly of fine chalky-looking
white or pale buff earths. These comprise a central mass
of pure Eadiolarian earth, passing both upward and down¬
ward through calcareo-siliceous earth into pure forami-
niferal marl or chalk. These beds are 230 feet thick, and
^bove them are red, pink, and yeUow oceanic clays.
^ " Voyage of the ' Challenger,' " vo\ i. p. 23L
CHAP. XTI.J DEEP-SEA AND OCEANIC DEPOSITS.
303
Diatom Ooze.—The lacustrine forms of the micro¬
scopic plants called Diatoms have already been mentioned
(p. 239). We are now concerned with those which live in
salt water. They abound in all seas, from the equator to
the poles, and have been found ahve in ice. According to
Ehrenberg, the siliceous cases of these organisms which
are annually deposited in the harbour of Wismar, on the
Baltic, amount to about 18,000 cubic feet, and the mud
there formed is consequently very siliceous. Ehrenberg
has also shown that the finest marine silts and muds, espe¬
cially such as are deposited in estuaries, contain large
numbers of Diatom frustules.
It is not surprising, therefore, to find that these siliceous
deposits form extensive beds in some parts of the world.
The town of Eichmond, Virginia, U.S., is buflt on siliceous
marls having such an organic origin, and forming strata
30 feet in thickness. In these beds the beautiful micro¬
scopic objects, Actinocyclus and Coscinodiscus are found, as
well as several species of Navícula and Galionella, some of
which are shown in fig. 73.
Sir John Hooker long ago mentioned their abundance in
the South Polar Ocean, and described a diatomaceous ooze
as occuring near the Victoria Barrier over an area measuring
about 400 miles in length and 200 in breadth. This was con¬
firmed by the " Challenger " observations, and it was found
that in passing from the tropical oceans to the great
Southern Ocean which surrounds the Antarctic continent
a change always took place in the character of the surface
fauna, the tropical species of the pelagic organisms gradu¬
ally disappearing and Diatoms becoming at the same time
more and more numerous. A corresponding change takes
place in the deposit on the ocean-fioor, the Foraminifera
becoming fewer and smaller and the percentage of Diatoms
increasing, till at length the deposit becomes a Diatom
ooze, which is yellowish when wet but white when dry, and
then resembles meal or flour. Of this material about 70
per cent, consists of Diatoms, either whole or in fragments,
and there is generally 10 or 20 per cent, of calcareous
shells, the remainder being clay and mineral fragments.
Rarity of Oceanic Deposits among Stratified
Rocks.—Although in this chapter some account has been
304
DYNAMICAL GEOLOGY.
[sec. ir.
given of the peculiar deposits which are now being formed
on the bed of the deep oceans, it is desirable to point out
that an acquaintance with these accumulations is of less im¬
portance to the geological student than a knowledge of the
mode in which deposits are produced in shallower waters.
His observations would soon teach him the close analogy
which exists between ancient shales, sandstones, and con¬
glomerates, and the most recent deposits of mud, sand,
and gravel which he can examine round our coasts, and he
Fig. 73. Diatoms from the Richmond Earth.
1. Navicula. 4,5. Actinocyclus (two species).
2. Coscinodiscus radiatus. 6. Coscinodiscus patera.
3. Galionella sulcata.
cannot fail to perceive that they must have been formed
under similar conditions. He will find also many beds of
limestone which are evidently made up of the debris of
corals, shells, and Echinoderms, and are therefore similar in
composition to the limestones which are now being formed
ip the neighbourhood of coral-reefs.
He might, however, travel over large tracts of the
earth's surface without finding any rock which is analo¬
gous in composition to that of the deep-sea muds above
described. The only rock in England which seems to
have been formed under oceanic conditions is chalk.
PART IL
STRUCTURAL GEOLOGY,
OR THE STRUCTURE AND RELATIVE POSITION
OP ROCK-MASSES.
The geological significance of the principal physical
operations which arc now taking place in or upon the
earth's crust has been considered in the first part of this
volume. The knowledge thus obtained will enable us to
explain the nature and structure of the various rock-masses
which make up that portion of the earth's crust that is
open to our observation ; and to understand how the rocks
have been brought into the positions in which they are
now found. This part of the subject includes two branches
of study : (1) the mineralogical composition of rocks ; (2)
the arrangement of rock-masses, and their relations to one
another.
When the student is in possession of these facts he is
able to enter upon a third branch of geological inquiry,
namely, that of Physiography, or the evolution of the
physical features of the earth's surface.
X
CHAPTEE I.
CBTSTALS AND EOCK-FOEMING MINEEALS.
IN the preceding pages we have assumed that the reader
possesses such an acquaintance with the commoner
kinds of rock as may he obtained from any elementary
treatise on Geology or Mineralogy. To comprehend what
has gone before it was only requisite that he should under¬
stand the difference between such common materials as
clay, chalk, sandstone, limestone, granite, etc. ; but before
entering upon the special study of rock-masses, it is cer¬
tainly necessary to have a more considerable acquaintance
with the different kinds of rocks and minerals which occur
in the earth's crust. The following chapters have therefore
been prepared with the object of imparting to the student
just so much mineralogical information as will enable him
to comprehend the contents of those that succeed them.
Petrology, or the study of the mineral components of
rocks, is based on the science of mineralogy. It is not
necessary, however, for geological purposes that the student
should make himself acquainted with all the different
minerals known to the mineralogist, because those which
may be considered as the essential constituents of rocks are
comparatively few in number. It is therefore only these
special rock-forming minerals which will be described in
the following pages.
Again, to understand mineralogy some knowledge of
chemistry is required, and if the student has not pre¬
viously acquired this, it will be desirable for him to read
some elementary treatise, such as Koscoe's "Primer of
Chemistry " (Macmillan's Series), or " Popular Chemistry "
("Whittaker's Series). Chemistry and Mineralogy being
308
STRUCTURAL GEOLOGY.
[part ii.
equally outside the scope of this manual, we shall not
occupy space by giving more than a brief summary of
what the student can learn elsewhere.
Chemical Elements and Compounds.—Chemical
analysis has discovered that there are between sixty and
seventy simple or elementary substances. All mineral
bodies and all organic bodies consist either of one of
these elements, or of a compound of two or more of
them. Except fifteen, these are all metals, and of these
metallic elements many are of very rare occurrence. Of
the other fifteen some are solid, as sulphur and carbon,
others are always found in a gaseous state, such as oxygen,
hydrogen, and nitrogen.
Of all these elementary substances not more than
eighteen enter into the composition of ordinary rock-
forming minerals ; these are given below, together with
the letters which chemists employ to denote them.
Non-Metals.
Oxygen O
Hydrogen H
Silicon Si
Carbon C
Boron B
Sulphur S
Chlorine CI
Fluorine F
Phosphorus . . . . P
Metab
Aluminium
Potassium.
Sodium. .
Calcium
Magnesium
Lithium
Iron. . .
Manganese
Titanium .
. Al
. K
. Na
. Ca
. Mg
. Li
. Fe
. Mn
. Ti
There are a great many other metals, and they are capa¬
ble of being grouped into classes, the members of each
class having certain characters in common; thus potas¬
sium, sodium, and lithium are metals of the alkalies ;
calcium and two others, strontium and barium, form the
alkaline earths ; and magnesium is so closely allied that
it might be put in the same class.
Oxygen is the most abundant element on the earth. It
combines with all the other elements except fluorine, and
these simple combinations, or oxides, are consequently the
most abundant and important compounds in nature.
Oxygen combined with hydrogen, in the proportion of one
atom of 0 to two atoms of H, forms water (HjO). With
CHAP. I.]
ROCK-FORMING MINERALS.
309
carbon it forms two compounds, carbonic oxide (CO) and
carbonic dioxide (COj). In combination witb silicon it
makes tbe substance silica (SiOj), and combined witb the
metals aluminium, calcium, magnesium, it forms alumina
(AljOs), lime (CaO), and magnesia (MgO). All these
substances are oxides, and when they have no special name
in ordinary use, like lime for the oxide of calcium, they
are spoken of simply as the oxide of iron, or oxide of man¬
ganese.
It will be observed that the proportion of oxygen varies
in different oxides, and some of the elements form two or
more oxides which differ in appearance and properties ;
thus there are three different oxides of iron—viz., FeO, the
protoxide; FcjOa, the peroxide; and FcjOj, an intermediate
oxide. In like manner carbon forms carbides, sulphur
forms sulphides, and chlorine chlorides.
Certain of * the oxides of metals, called hasic oxides,
combine with water to form compounds called basic
hydrates, e.g. lime (calcium oxide) with water forms slaked
lime (calcium hydrate) ; potassium oxide similarly yields
potash (potassium hydrate). The basic oxides and their
hydrates are collectively termed bases.
Most of the oxides of the non-metallic bodies, on the
other hand, combine with water to form compounds, called
acid hydrates or acids, analogous in their composition to
basic hydrates, but differing in their chemical properties.
The simple acid-forming oxides are in this relation called
anhydrides, i.e. bodies without water. Sulphuric acid
(H2SO4) and carbonic anhydride (CO^) are examples of an
acid hydrate and anhydride respectively.
These acids unite with bases to form compounds termed
salts, water being at the same time produced. Salts are
neither acid nor basic in character, and their formation
consists in the replacement of the hydrogen of the acid by
the metal of the base, the hydrogen thus set free uniting
with the oxygen of the base to form water. The salts of
the acids above mentioned are denoted by changing the
termination " ic " of the acid into " ate," as carbonate, sul¬
phate, silicate ; thus, sulphuric acid and magnesia unite
to form magnesium sulphate and water. In this case,
sulphuric acid is the acid, magnesia is the base, and mag-
310
STRUCTURAL GEOLOGY.
[part ii.
nesium sulphate the resulting salt. The reaction is expressed
graphically thus :
Binary compounds, or the unions of two elements only,
are then designated by the termination ide, and ternary
compounds or unions, of at least three elements, are
designated by the termination ate.
Salts can also be formed by the action of acid anhydrides
on basic hydrates ; and it is in this way that glass and
other silicates (salts of silicic acid) are produced. Thus, if
silica is heated to a high temperature with potash or soda,
they will unite, and form a hquid which, on being cooled,
solidifies into the transparent substance known as glass.
The silicates of alumina, magnesia, lime, and iron are the
principal constituents of one class of rocks.
Crystallography.—A crystal is a regular geometrical
solid, the external form of which is dependent on its inter¬
nal structure. All crystalline bodies are so symmetrically
constructed that they generally have a tendency to split
in certain directions, and this splitting is called crystalline
cleavage. The surface planes of crystals have definite
relations to certain imaginary lines called axes, which are
assumed to intersect in the centre of the crystal, and the
portions of these lines which are cut off by the surface
planes of a simple crystal are called parameters. Thus
the lines which would touch opposite faces of a cube and
intersect one another are parameters. It is generally
sufficient to select three of these symmetrical axes to
express the ordinary relations of length, breadth, and thick¬
ness ; but sometimes it is necessary to select four axes.
H,S04 + MgO = MgSO, + H,0.
Fig. 74.
Fig. 75.
Fig. 76.
CHAP. I.J
ROCK-FORMING MINERALS.
311
The following are the six systems of crystallization which
are ordinarily adopted :—
1. Isometric or Cubic System.—Three parameters, all
equal, and at right angles. Examples : The cube, with six
square faces, as in fluor spar (fig. 74) ; and the octahedron,
with its sides equilateral triangles (see fig. 75).
2. Tetragonal or Pyramidal System.—Three parameters
Fig. 77. Fig. 78. Fig. 79.
at right angles, but only two of them equal. Examples :
The rectangular prism on a square base (fig. 76) ; and the
octahedron, with its sides isosceles triangles, as in fig. 77.
8. Hexagonal or Rhomhohedral System.—Four para¬
meters, three equal and crossing each other at angles of
60° in the same plane, the other perpendicular to this plane.
Examples; The six-sided prism on a hexagonal base (see
fig. 78), or terminated by a six-sided pyramid, as in quartz.
«
Fig. 80. Fig. 81.
4. Rhombic or Prismatic System.—Three unequal para¬
meters at right angles. Examples : The rhombic prism
(fig. 79), and the rhombic octahedron (fig. 80).
5. The Monoclinic System.—Three unequal parameters,
two at right angles, and one at right angles to one only of
these. Example: The oblique rhombic prism (fig. 81).
6. Triclinic or Anorthic System.—Three parameters, all
312
STRUCTURAI GEOLOGY. [PAET II.
unequal, and oblique to each other. Example : The doubly
oblique prism.
The diversity of forms presented by natural crystals is
almost infinite ; but they may all be referred to one or
another of these systems, and may be actually produced by
a modification of one of the typical forms above mentioned.
Thus, in the first system, a regular octahedron can be ob¬
tained from a cube in the following manner :—If the eight
comers of a cube be regularly truncated, or cut off, as in¬
dicated in fig. 82, eight new faces will be produced, resulting
in fig. 83 ; and if the process is continued by cutting away
equal portions from these new faces, until the planes meet
or touch each other, the regular octahedron will be deve¬
loped, as shown inside fig. 84.
Similarly, other varieties of form may be produced by
the partial or complete truncation of the sides of a cube or
octahedron. Thus, if the twelve edges of an octahedron be
truncated, we have the form of fig. 85 ; and, if this trunca¬
tion be continued tül the planes intersect, it wiU result in
the formation of a regular twelve-sided figure, called the
rhombic dodecahedron (fig. 86).
But, if the eight comers or angles of the octahedron are
truncated, the cube is produced, as shown in fig. 87. Thus,
CHAP. I.J ROCK-FORMING MINERALS.
313^
all three forms may be developed from each other ; and
similar relations exist between the forms of other systems.
Twinning of Crystals.—Crystals are frequently found
which do not appear to belong to any of the regular types
above mentioned, or to any direct modification of them.-
They are, in fact, compound crystals, that is, portions of
two or more crystals which have grown together, so that
each one is imperfect. An ordinary twin crystal consists
of two half-crystals united along a certain plane, but in
such a manner that their crystalline axes and faces do not
correspond. Such a dual crystal can be formed by cutting
a complete crystal in half, turning both halves half round
a certain axis (i.e. each through 90°) and reuniting them ;
the plane along which they are reunited is called the com¬
position plane, and the plane parallel to the axis of turning
is called the tvnnning plane. Some form an angle with
each other like a knee-joint, and are hence called genicular
twins ; and double genicular twins, thus composed of four
half-crystals, sometimes form perfect crosses, which are
characteristic of certain minerals.
Other crystals consist of a great number of partially
formed crystals all united along parallel planes, this is
termed polysynthetic twinning. Such repeated twinning
often produces a striated appearance when the crystal is
broken along a certain plane of cleavage, and it is always
visible when a slice of the crystal is viewed under a micro¬
scope with polarized light.
Rock-forming Minerals.—We now proceed to de¬
scribe those minerals which are the essential constituents
of rock-masses. Of the simple elementary substances men¬
tioned on page 308, two only are ever found as minerals, viz.,,
carbon and sulphur. The latter never occurs as a rock-
constituent, though it is sometimes rather abundant near
volcanoes ; the former occurs as a constituent of coal, and
as graphite in schist and gneiss. All other minerals are com¬
pound substances, and are either binary or ternary combi¬
nations, which may be classified in the following manner:—
Binary compounds : A. Oxides, chlorides, fluorides.
Í B. Carbonates, sulphates, phos-
Ternary compounds < phates.
( C. Silicates.
314
STRUCTURAL GEOLOGY.
[part II.
A.—Minerals formed of Binary Compounds.
Silica, when pure and crystallized, is known as Quartz,
its separate crystals being often called rock-crystal, and
sometimes Irish diamond, Bristol diamond, etc. The
specific gravity of quartz is between 2 6 and 2*7.^ The
crystals belong to the hexagonal system, their most usual
form being a six-sided prism terminated by six-sided pyra¬
mids ; they are sometimes very large, and sometimes so
minute that the mineral appears to be a compact hard stone,
which is milk-white when pure. Amethyst, agate, chalcedony,
■carnelian, onyx, cat's-eye, are aU mere varieties of quartz,
stained of various hues by slight admixtures of iron, man¬
ganese, or other colouring matter.
Jasper and Bloodstone are impure forms, rendered
npaque by the presence of oxides of iron ; jasper is gene¬
rally red, and bloodstone green spotted with red.
Opal and its varieties are hydrated and non-crystalline
forms of silica. One of these varieties, called hyalite,
occurs in the form of minute globules in some sedimentary
rocks, and another, called menilite, occurs in the form of
concretionary nodules, like the flints in chalk. The
hydrated form of silica precipitated fiom thermal waters
is called sinter or geyserite; its mode of occurrence has
Already been described on p. 206. The specific gravity of
opal is 2"2.
Flint and Chert appear to be mixtures of opaline and
chalcedonic silica in varying proportions, but the opaline
clement is often obscured by the crystallization of the
chalcedony.
When quartz occurs in an igneous rock it is usually a
greyish, semi-transparent, glassy-looking, irregularly crys¬
talline granule, which cannot be cut or scratched by even
the best steel knife, and has no definite cleavage planes.
Oxide of Iron.—Two oxides of iron occur as minerals,
the peroxide or sesquioxide (Fe^Oa), and the less highly
oxidized Fe304.
' The specific gravity of a substance is its weight compared with
that of the same bulk of pure distilled water at a temperature of
«0° F.
CHAP. I.]
ROCK-FORMING MINERALS.
315
Hœmatite (Fe^Oj) is red or black in colour, but always
gives a blood-red streak. When distinctly crystallized it
is called specular iron, and belongs to the hexagonal
system, and usually occurs in thin tabular crystals, or
scales. It often forms fibrous nodules, but many of the
beds and nodules of red iron ore which occur among the
stratified rocks have resulted from the decomposition of
carbonate of iron by the action of water, and haematite is
therefore frequently a pseudomorph of chalybite.
Limonite is a hydrated form of haematite, with the for¬
mula 2Fe203, 3HjO, and generally of a yello-wish-brown
colour. Yellow ochre and bog-iron ore are varieties.
Magnetite (FcaOi), so called from its magnetic properties,
is usually black in colour and streak, and crystallizes in the
cubic system, generally in some form of the octahedron.
It is frequently met with in igneous rocks, where it is dis¬
seminated in small crystals, always black and opaque, and
presenting square or triangular sections under the micro¬
scope.
Ilmenite, a mixture of the oxides of iron and titanium,
crystallizes in rhombohedral forms, and often occurs in thin
plates or tables, the outer parts of which are frequently
decomposed into a greyish-white substance (Leucoxene).
Iron pyrites, or sulphide of iron (FeSa), crystallizes
sometimes in the isometric and sometimes in the rhombic
system ; when in the latter, it is called marcasite. It is
generally of a light bronze colour, and is- found dissemi¬
nated in rocks of many kinds and all ages. In igneous
and metamorphic rocks, it occurs in small, sparkling cubic
or octahedral crystals. In stratified rocks, the marcasite or
rhombic forms are, perhaps, more abundant, especially in
dark shales and clays. According to Ebelmen, pyrites is
always formed wherever organic matter in process of de¬
composition, acts on the sulphates of sea-water in the pre¬
sence of ferruginous mud. Thus fossil organisms are often
found coated or filled with iron pyrites ; and the radiated
globular nodules of marcasite frequently contain some
small organic nucleus.
Rock salt, or sodium chloride (NaCl), crystallizes in
the isometric system, and generally in cubes ; it is white
when pure, but is usually coloured red, blue, or purple.
316
STEUCTUKAL GEOLOGY,
[PAET II.
Eock salt occurs in extensive beds, wbicb are sometimes of
great thickness.
Fluor spar, or calcium fluoride (CaFj), crystallizes in
the isometric system, and usually in cubes, which are very
variable in colour. It occurs chiefly in veins, and can hardly
be called a rock-constituent.
B.—Carbonates and Sulphates.
Calcite or calc-spar, carbonate of lime (CaCos), is,
after qnartz, the most abundant mineral in nature. Crys¬
tallized in the rhombohedral system, with a specific gravity
of 27, it is called calcite ; but it also forms rhombic
crystals, and is then known as aragonite, which is harder
and heavier but more easily soluble than calcite. Both
effervesce strongly when touched with hydrochloric acid,
and this is therefore the usual test for calcareous matter in
a rock. Calc-spar always cleaves readily into rhombo-
hedrons ; and the name of Iceland-spar is given to such as
are clear, and are consequently used to exhibit the property
of double refraction.
Stalactites and stalagmites are formed of successive con¬
centric films precipitated from evaporating water, and the
radiating crystalline plates which traverse these concentric
coats, without obliterating them, show that the crystalliza¬
tion has been subsequent to their formation. Dolomite is
a compound of the two carbonates of lime and magnesia ;
and when the mixture is crystallized, it is known as bitter-
spar, with a specific gravity of 2'85.
Calcic sulphate, or anhydrite (CaSOJ, is better known
in the hydrated form of Gypsum (CaSOj, 2H0), which
usually crystallizes in monoclinic prisms. Gypsum, when
transparent, is called selenite ; when fibrous, satin-spar;
and some of the compact varieties are popularly called
alabaster. Gypsum is harder than calcite, and slow to
effervesce with acid. It is only found in sedimentary rocks,
occurring sometimes in veins and nodules, and sometimes in
beds of great thickness, associated with rock-salt. Its for¬
mation from the evaporation of sea-water has already been
noticed (p. 247). Crystals of selenite are abundant in
many clays, either single, or in radiating groups.
CHAP. I.]
ROCK-FORMING MINERALS.
317
Carbonate of Iron, often called Chalybite or Siderite, is
a carbonate of the protoxide of iron (FeCOj), and is another
common mineral. It occurs crystallized in veins and beds
among crystalline rocks and in the older limestones, having
a dull yellow or brownish colour, and a pearly lustre on the
cleavage faces. In an earthy state, and mixed with a cer¬
tain amount of clay or silt, it forms the well-known clay-
ironstones which which are so largely worked for iron ore.
The heavy brown nodules and septarian stones so common
in clays generally consist of mixtures of the carbonates of
iron and lime.
Phosphate of Lime is generally found as a tribasic salt,
with the formula of CajPOi. When crystallized, it is called
apatite, and occurs in six-sided prisms or tables, belonging
to the hexagonal system. Small crystals are not unfrequent
in igneous and metamorphic rocks. It is also found mas¬
sive, forming beds several feet thick.
The phospate nodules, so frequently found in stratified
clays and marls, contain from 30 to 60 per cent, of tribasic
phosphate of lime, mixed with carbonate of lime and other
earthy materials.
C.—Silicates.
1. The Felspar Group.
The Felspars play a very important rôle in the consti¬
tution of rocks. They all consist of silicate of alumina,
combined with the silicates of potash, soda, or lime. The
names of the most important Felspars are given in the fol¬
lowing table, together with their average composition :—
Silica.
Alumina,
Potash.
Soda«
Lime.
Sp. Gravity.
Orthoclase .
64-6
18-4
16-9
_
_
2-55
Microcline .
64-3
19-6
15-6
—
—
2-54
Albite. . .
68-6
19-5
—
11-8
—
2-62
Oligoclase .
62-1
23-7
—
10-2
4 0
2-64
Ândesine. .
59-7
25-6
—
7-7
7 0
2-66—2-69
Labradorita .
52-9
30-3
—
4-5
12-3
2-68—2-74
Anorthite .
431
36-8
" ~
201
2-75
318
STRUCTURAL GEOLOGY. [PAET II.
Orthoclase and Microcline are, therefore, Potash Felspars;
Albite is a Soda Felspar ; OUgoclase and Andesine are Soda-
Lime Felspars ; Labradorite is a Lime-Soda Felspar ; Anor-
thite is a Lime Felspar. The first two are the lightest, and
the last is the heaviest. One only (Orthoclase) crystallizes
in the Monoclinic system, all the others belonging to the Tri-
clinic system ; hence the group is sometimes divided into the
Orthoclastic and the Plagioclastic Felspars. It has been
supposed by some that Oligoclase, Andesine, and Labra¬
dorite are not distinct species, but mixtures of Albite and
Anorthite in varying proportions.
Orthoclase Felspar generally occurs as a translucent, or
semi-opaque, mineral, of a white or flesh-red colour, in
monoclinic crystals of an oblong shape, which are frequently
twiimed. When transparent, with a pearly opalescence, it
is called Adularia ; Sanidine is a similar transparent and
glassy variety, forming crystals of a tabular shape, and
chiefly occurring in volcanic rocks.
Microcline is closely allied to Orthoclase, even the external
crystalline form being similar, though it is really triclinic,
and exhibits polysynthetic twinning. Under a microscope
and -with polarized light fine bands are visible, which cross
one another and produce a characteristic pattern.
Plagioclase Felspars of the Soda-Lime series generally
form long and narrow crystals, with a bright shining sur¬
face, and are usually white or greenish grey. Under the
microscope, they may generally be distinguished from
Orthoclase by the presence of fine parallel strise (due to re¬
peated twinning) along some of the planes of cleavage.
These are never seen in Orthoclase.
Albite is generally white, sometimes clear and sometimes
dulled by partial kaolinization. It occurs separately in
some granites, but more often in bands, as in Perthite, or in
zones round orthoclase.
Oligoclase and Andesine are usually whitish minerals,
with a more or less vitreous lustre. They are generally
composed of a number of long twinned crystals, which cause
a well-marked parallel striation.
Labradorite is the commonest Triclinic Felspar, and forms
masses of long twinned crystals, like those of Oligoclase ;
but usually grey, green, or brownish in colour, with a
CHAP. I.]
ROCK-FORMING MINERALS.
vitreous or pearly lustre. Labradorite is sbghtly soluble-
in hydrocbloric acid ; but, under the microscope, it is.
difficult to distinguish it from Oligoclase, though the
former often exhibits a number of fine parallel lines, which
cut obliquely across the stripes of colour seen by polarized
light.
Anorthite is a mpLch rarer species, though it sometimes,
occurs in volcanic rocks, such as the Vesuvian lavas. Being
a very low silicated Felspar, it is soluble in hydrochloric
acid with evolution of free silica.
Nepheline and Leucite are light-coloured minerals
closely l-elated to the Felspars, but crystallizing in different
forms. The one occurs in rather stumpy six-sided prisms,,
and the other in icosi-tetrahedrons or solids -with twenty-
four faces. Nepheline is essentially a silicate of alumina
and soda with a little potash, while Leucite is a sUicate of
alumina and potash. Nepheline has a specific gravity of
2'6 ; while that of Leucite is 2-5. Leucite is common" in
the lavas of Vesuvius, but has not been found in England,,
and Nepheline only in the rock called Phonolite.
2. The Mica Group.
The Micas are a group of minerals which vary much,
in composition, but have several properties in common.
The crystals, whether belonging to the monoclinic or hexa¬
gonal system, split readily into very thin flexible plates,
which have a bright pearly lustre and are more or less-
transparent. They are composed of a mixture of silicate of
alumina with silicates of potash, magnesia or lithia, and
some iron. They are rather heavy minerals, their specific
gravity being from 2'75 to 3"2, and therefore heavier than
the Felspars. They vary in their degree of fusibility, but
are all easily scratched with a knife.
Muscovite, or Potash Mica, usually occurs in six-sided
tabular crystals of a grey greenish or yellowish colour. It.
is very clear and transparent, and often forms large foliated
masses, the separate plates of which are used instead of
window glass in Siberia and elsewhere. Commercially it is
known as talc, and is used for lamp-shades, etc., but the
true talc is a different mineral. Muscovite is very liable to>
320
STRUCTURAL GEOLOGY.
[PAET II.
hydration, and then passes into the white silvery-looting
variety called Margarodite. Sericite is another hydrated
mica, silvery-white or pale green, which occurs chiefly in
certain metamorphic rocks.
Lepidolite, or Lithia Mica, may be regarded as Muscovite
in which the potash is partially replaced by lithia. Its
physical properties are similar and its crystalline form the
same, but it is usually rose-red or violet in colour, and
commonly forms granular masses consisting of numerous
scales or folia.
Biotite, or Black Mica, is a silicate of alumina, magnesia,
potash, and iron, generally dark green or black. Lepido-
melane is a variety with less alumina and magnesia, and
more iron. It is found in some Irish and Cornish granites,
and its colour is black.
3. Hornblende and Pyroxene Group.
Hornblende and Augite.—Under these names are in¬
cluded a number of heavy | minerals which are closely
related to one another, their essential composition being a
double silicate of magnesia and lime, the bases being often
more or less replaced by iron or manganese. Alumina is
often present. They all crystallize in the monoclinic system.
Common Hornblende is a dark green or greenish black
mineral usually occurring in long prisms. Its specific
gravity varies from 3 to 3"47. The crystals frequently
have a fibrous structure and a silky lustre, with external
angles of about 60° and 120°, properties which serve to
distinguish them from crystals of Augite.
Aii^ite generally occurs in short stout prisms, with
external angles which are nearly at right angles to one
another. The crystals are usually black or brown, but
there are whitish and green varieties, known respectively by
the names of JDiopside and Sahlite. Its specific gravity is
from 3'3 to 3"5.
Pyroxene is another name for Augite, and Amphibole for
Hornblende. There is good reason for supposing that they
are identical minerals crystallized imder different circum¬
stances. Augite has been artificially formed in slags which
-cooled rapidly. Hornblende has never been so obtained.
CHAP. I.J
ROCK-FORMING MINERALS.
321
and always occurs in rocks which must have cooled slowly.
Hornblende has been observed in the thicker parts of a vein
of lava exposed in the Val del Bove, Etna, while Augite
occurs in the thinner parts of the same vein. Lastly,
there is a mineral called TJralite, which has the outward
form of Augite, but the cleavage and structure of Horn¬
blende, and Augite can be observed under the microscope
in various stages of change into Hornblende.
Diallage is a variety of augite, with a peculiar cleavage,
and generally having a pearly grey or brownish lustre ;
Tremolite, ActinoUte, and Asbestos, are more or less fibrous
varieties of Hornblende, containing little or no alumina.
Hypersthene, Enstatite, and Bronzite are closely allied
minerals, but differ in not containing lime, and in crystal¬
lizing in the rhombic system. They are all silicates of
magnesia and iron: the first two are generally of a dark
greyish brown colour ; the last is a dark yellow or brown
with a bronze-like lustre, and has much resemblance to
Diallage. Its specific gravity is from 8"1 to 8'8.
The following are average analyses of some of the above
minerals, but the proportions are not always the same :—
Dark
Hornblende.
Actinolite.
Augite
(Diopside).
Bronzite.
Silica
46
56
51
56
Magnesia . . .
16
24
14
30
Lime
13
13
21
—
Alumina . . .
12
—
1
2
Iron . . . .
12
7
12
12
4. Peridote Group.
Peridote is a silicate of magnesia and iron, and is,
therefore, chemically related to hypersthene and bronzite,
but it contains much less silica, so that the relative propor¬
tions of silica and magnesia are generally nearly the same.
It crystallizes in the rhombic system, and when found in
transparent rhombic prisms it is often called Chrysolite,
T
322
STRUCTURAL GEOLOGY.
[PAKT II.
but it is much more abundant in the form of grains or
granular lumps of an olive or yellowish green colour ; it is
then known as Olivine, and is of frequent occurrence in
certain igneous rocks, sometimes in such quantity as to be
the chief constituent of the rocks called Peridotites.
Serpentine is a hydrated silicate of magnesia and iron ;
it is generally compact and massive, of a dark green colour,
and is greasy to the touch ; but it varies much in colour,
and is sometimes yellow, brown, red, or black, from the
presence of iron oxides. Serpentine, like steatite, results
from the decomposition of magnesian minerals, such as
Olivine and Bronzite, and gives its name to a particular
kind of rock, which is almost entirely composed of it.
The average general composition of Olivine and Ser¬
pentine is given below for the purpose of comparison :—
Olivine.
Serpentine,
Silica
40-2
SB'S
Magnesia
417
38-5
Oxides of iron....
181
10-2
Combined water . . .
—
12-8
5. Silicates which generally occur as Secondary or Accessory
Minerals.
Andalusite is essentially a silicate of alumina, though
it is seldom quite pure. It crystallizes in the rhombic system, '
and the larger crystals are often encrusted with and pene¬
trated by scales of mica, which may be a product of its
decomposition. The variety Chiastolite, so called from
the cruciform markings in the interior of the crystals, is
usually met with in slates which have been partly meta¬
morphosed by contact with, or proximity to, igbeous rock.
Cyanite is a pure silicate of alumiua generally of a pale
blue colour.
Staurolite is another alteration product, crystaUizing
in rhombic prisms which are frequently double genicular
CHAP. I.J KOCK-FORMING MINERALS.
323
twins, forming a cross, whence its name. It consists of the
silicates of alumina and iron.
Garnets consist of silicate of alumina with a silicate of
some other base, such as lime, magnesia, iron, and man¬
ganese. The percentage of silica is always from 35 to 40,
and all the varieties crystallize in the cubic system, and
usually in rhombic dodecahedrons. They are generally of
a dark colour, red, brown, or black, and are of common
occurrence in metamorphic and intrusive igneous rocks.
Idocrase has a nearly similar composition, but crystallizes
in tetragonal forms, is usually hghter in colour, light brown,
jellow, or greenish white.
Epidote is a silicate of alumina, lime, and iron ; it
crystallizes in the monoclinic system, and usually forms
long blade-hke crystals in radiating groups or nests of a
light green colour. It is essentially an alteration product.
Hauyne and Nosean are substantially silicates of
alumina and soda, with some sulphate of lime or soda ;
and are related to the garnets. The first is usually of a
bright blue colour, and the second of a yellowish grey.
Tourmaline, or Schorl, may be regarded as a combi¬
nation of a silicate of alumina and iron with a borate.
The chemical composition is variable, but it generally con¬
tains : sihca, 40 per cent. ; alumina, 30 to 40 per cent. ;
boracic acid, 3 to 4 per cent. ; with peroxide and protoxide
of iron, and protoxide of magnesia. In colour it is greenish
or reddish-black, and somewhat resembles hornblende, but
may be distinguished by its more resinous fracture, the
Absence of distinct cleavage, and its greater hardness.
Chlorite appears to be a hydrated silicate of magnesia
And alumina, part of the latter being replaced by ferrous
oxide ; like all products of alteration, its chemical compo¬
sition varies, and, like mica, it is the name of a group
rather than of a single mineral; it generally occurs in
•earthy or scaly granules of a leek-green colour ; sometimes
it forms fibrous radiating crystals.
The Zeolites are hydrated double silicates having
many varieties, to which separate names are given ; most
of them consist of silicate of alumina with silicates of
hme, soda, or potash. They are, therefore, closely related
to the Felspars, and may indeed be regarded as hydrated
324
STRUCTURAL GEOLOGÏ. [PAET II.
varieties of the Felspar group. The name Zeolite is
derived from their intumescence or boiling up under the
blow-pipe as the water is driven off. Mesotype, Prehnite,
Chabasite, Analcime, Stilbite, and Thomsonite are among
those most frequently met with.
Glauconite is a hydrated silicate of iron, alumina, and
potash, of a dull green colour ; it is found in cavities of
igneous rocks as a result of decomposition, and also occurs
iu the form of loose grains in marls and sandstones.
Q-lauconite grains are of frequent occurrence in rocks of
all ages, and certain beds called Greensand consist of sandy
or marly matter in which so many glauconite grains are
distributed as to impart a green colour to the mass.
Ehrenberg long ago demonstrated that most of these
grains were casts of Foraminifera, and this is confirmed
by the observations above mentioned. It is possible that
some of the larger and more irregularly-shaped grains may
be fragments of the casts formed in larger shells, such as
some of the Pteropodous Mollusca ; these shells always
accompany those of the Foraminifera on the ocean bottom,
and their casts in green earth have likewise been met
with.
Mr. Sorby, in the course of his microscopical investiga¬
tions, has found glauconite in the cells of Foraminifera,
in the hollow spaces in the tissue of corals and Echmo-
derms, in holes bored into shells, and, in fact, in all kinds
of cavities. Sometimes it occurs in the form of rounded
grains or angular fragments which seem to have been
formed elsewhere and drifted as sand into their present
position.
Talc is a hydrated silicate of magnesia, and its per¬
centage composition is generally as below :—
When crystallized it is ortho-rhombic, but it usually occurs
in a massive condition, splitting into thin laminse, which
are flexible, but not elastic as the plates of mica are. In
colour it varies from white to apple-green. Steatite or
Silica
Magnesia
Water
60 to 62.
30 to 33.
Ito 7.
CHAP. I.J ROCK-FORMING MINERALS.
325
Soapstone is a coarse earthy variety of talc, usually of a dull
white or yellowish colour, and always found in a massive
form. Both talc and steatite have a soft soapy feel.
Meerschaum is a silicate of similar composition.
Pseudomorphs.—Pseudomorphism is the occurrence
of one mineral in the crystalline form of another mineral.
This may take place either by alteration or substitution.
In the one case, the first mineral has been gradually
changed into another by the addition or exchange of con¬
stituents. In the second case, the new mineral is formed
by the gradual replacement of the first mineral particle by
particle. Both processes are accomplished by the action of
carbonated water, containing solutions of various minerals
obtained from the rocks through which it has percolated.
Penetrating deeply into the earth, acquiring a higher tem¬
perature, and helped by pressure, such water becomes an
active agent in altering the constitution of minerals and
rocks.
Some of these changes have been imitated in the labo¬
ratory of the chemist. Thus " Stein converted a crystal
of gypsum (sulphate of lime) into carbonate of lime by
leaving it for several weeks in a solution of carbonate of
soda at a temperature of 122° P." ^ The sulphuric acid of
the gypsum united with the soda to form sulphate of soda,
which was dissolved and carried away by the water, while
the lime united with the carbonic acid, and remained in the
crystalline form of gypsum. The result is only attainable
under certain conditions, of which the most essential is
slow action, otherwise the original form is lost ; but these
conditions are eminently fulfilled in all natural operations.
Again, Calcite is frequently found as a pseudomorph after
other minerals.
The process by which such pseudomorphs are produced
is a very important one, since it is precisely that of petri¬
faction, or the mode in which organic forms are mineralized,
and their external form, with more or less of their internal
structure, preserved for our examination.
^ Bischof, "Chemical Geology," vol. i. chap. ii.
CHAPTER II.
CLASSIFICATION OF EOCKS.
Meaning of the word Rock.—Before attempt¬
ing to classify the various kinds of rocks which
enter into the composition of the earth's crust, it is neces¬
sary in the first place to understand what geologists mean
by the term rock. In ordinary language a rock means
a hard and massive stone, but geologists know that the
character of hardness is an accidental one. The very same
beds of limestone may be soft chalk in one place and hard
marble in another. The same beds of clay may be harder
than the hardest brick in one place, and soft enough to
mould into bricks in another ; and a sandstone which is
hard grit in one part, in another be dug out with a spade.
Geologists apply the word "rock," then, as a general
term to any considerable mass of mineral matter, whether
hard or soft, or whatever may be its form or character,
provided it be of suflicient importance to be spoken of as a,
constituent part of the crust of the earth.
A rock, therefore, may be defined as a mass of mineral
matter, consisting of numerous crystalline or fragmentary
particles which may belong to one or several different kinds
of minerals, and which may be loosely coherent or firmly
compacted together.
Texture and Structure of Rocks.—Every kind of
rock possesses a special structure and texture ; its struc¬
ture depending on the nature and character of its compo¬
nent particles, its texture on the comparative size of these
particles, and the manner in which they are arranged.
These qualities of a rock can he determined from small
CHAP. II.J CLASSIFICATION OF ROCKS.
327
samples or hand specimens, and the study of them is a
part of Mineralogy or Lithology.
When rocks are considered as rock-masses, and studied
in their natural positions, other points of difference present
themselves ; the manner in which they occur, and the shape
of the layers, blocks, or masses into which they split, are
found to depend largely on the manner in which they have
been formed änd the materials of which they consist. The
study of these relationships is that of Field G-eology or
Geognosy.
Lithological Classification of Rocks.—Itisevident,
then, that rocks may be classified either according to their
internal structure or their external relations. Let us, in the
first place, see to what conclusions a lithological arrange¬
ment would lead us. The words in which we have defined
a rock suggest a primary lithological difference, whereby
all rocks may be divided into two classes. Some are com¬
posed of definite crystalline particles, which, if not perfect
crystals, yet possess some of the external faces and angles
of perfect crystals, and have clearly been formed in the
position which they now occupy. With these may be
grouped certain rocks which consist partially or wholly of
a glassy substance. Other rocks consist of a congeries of
particles which have not grown together, but are fragments
which have been broken off their parent masses and brought
together by some external agency; their coherence being
caused either by mechanical compression or by a cement
of some other substance.
By these lithological differences, therefore, rocks may be
primarily distinguished into ; 1, Crystalline ; 2, Frag¬
mentary.
Crystalline Rochs.—" All crystals are built up by the
successive external addition of minute crystalline particles.
It is clear, then, that these particles must have been free
to move and arrange themselves ; in other words, they must
have been the result either of solution in water or other
liquids, or of fusion by heat. Whenever, then, we find a
crystal or a mineral particle that has an internal crystalline
structure, we may feel assured that this structure has been
produced either by solution or fusion ; in other words, that
the crystal has been either dissolved or melted. But if
328
STßUCTUßAL GEOLOGY.
[PAET II.
this be true as regards individual crystals, or crystalline
particles, it must also be true of rocks that are made up of
such crystals or particles." ^
It has been already stated (see p. 85) that some minerals
are soluble in water containing carbonic acid, and that
under certain circumstances they may be deposited directly
from such chemical solution. Other minerals, again, such
as those which compose volcanic rocks, though practically
insoluble in water, can be made fluid by the influence of
heat. If then rapidly cooled they solidify into a glass, if
slowly they form a crystalline mass. Crystalline rocks
therefore are all chemically-formed, and are divisible into
two sections : a. Aqueous crystalline rocks, and, h, Igneous
crystalline rocks.
2. Fragmentary Bocks.—These consist of particles which
have been derived from the disintegration and detrition of
pre-existent rocks, or from the decay and disintegration of
organic bodies. They may be divided into two groups or sec¬
tions, accordingto the manner of their formation, viz. : a. Me¬
chanically-formed rocks ; and h. Organically-formed rocks.
The particles of mechanically-formed rocks bear evident
marks of mechanical fracture and attrition, most of them
having been more or less rounded and worn by currents of
water or of wind. From the broken, chipped, and rounded
aspect of their component particles, such rocks are often
termed Clastic, from KXaaroç, broken. This detritive and
fragmental origin is very clear in the case of such rocks as
are chiefly made up of pebbles or rounded fragments of
other rocks, and is hardly less obvious in the case of sand
and sandstones. Even igneous rocks have their mechanical
accompaniments, in the shape of the dust, ashes, and frag¬
ments ejected from volcanic craters, and these may be
compacted into solid rocks, whether they fall on the land
or into the water.
The organically-derived rocks are wholly or in great
part composed of fragments of the hard structures secreted
by certain plants and animals, and the manner of their
formation has already been fully described. The frag¬
ments may be little altered from their original condition.
' Jukes' "Manual of Geology," second edition, p. 50.
CHAP. II.J CLASSIFICATION OF KOCKS.
329
or else they maybe greatly altered and partly mineralized.
In the latter case they become allied to the aqueously-
formed crystalline rocks.
A lithological arrangement of rocks therefore results in
our making four groups, thus :—
This, however, is not quite satisfactory, for it classes to¬
gether aqueous and igneous rocks, and separates the
chemically-formed aqueous rocks from their closely alhed
neighbours of organic origin. It is evidently, then, desir¬
able that we should ascertain whether rocks do not ex¬
hibit some differences which are more closely in relation
to the conditions under which they have been formed.
Tectonic Classification of Rocks.—Eocks may be
classified by the differences in the tectonic structure, or
the manner in which they are built up into rock-masses.
Those which have been deposited as sediments on the
bottoms of seas, lakes, and rivers, are arranged in regular
layers, beds, or strata. Those rocks, on the contrary, which
have consolidated from a state of igneous fusion are
always unstratißed and massive in their structure. Hence
we have a primary distinction into Stratified and Unstratified
rocks, and it will also be found that most of the stratified
rocks are fragmental (and derivative), while most of the
unstratified rocks are crystalline.
Certain rocks will, however, be met with, which present
a combination of these characters, possessing a more or
less crystalline texture, yet often showing signs of having
once been arranged in regular strata. Other rocks exhibit
a peculiar fissile structure, which causes them to split
along planes which are more or less oblique to the planes
of bedding. It is natural, therefore, to conclude that such
rocks have undergone a considerable amount of alteration
since the period of their original formation, and that new
structures have been superinduced upon that of stratifica¬
tion. This kind of alteration is called Metamorphism, and
the rocks so affected are called Metamorphic rocks.
330
STRIICTURAL GEOLOGY. [PAET ii.
A tectonic classification therefore, founded upon the
structural differences of rock-masses, is a more natural and
convenient system of arrangement than that based upon
their textural and mineralogical composition. It suggests
a twofold division into—1. Stratified rocks ; 2. Unstrati-
fied rocks, each of these primary groups having a sub¬
group of Metamorphic rocks.
Genetic Classification of Rocks.—Another basis for
the classification of rocks may be found in a primary dis¬
tinction betvreen (a) rocks which have been found on the
surface of the globe by means of the various agencies which
have been described in Part I., and (b) rocks which were
part of the liquid magma underlying the earth's crust, and
have therefore been brought up from below that crust.
It is clear, too, that all the rocks which were formed on
the surface during any given period of time, must have
been derived from the rocks which existed before that
time ; and that these must have been either Igneous
rocks, or older stratified and Clastic rocks. It is true
that fresh supplies of Igneous rock have been brought up
from time to time, but the amoimt of such material
erupted during any one period has never been so great as
the amount of sedimentary rock formed during that period.
Hence in each succeeding period of geological time the
quantity of derived and stratified rock must have increased ;
and inversely, during the earlier portion of the earth's his¬
tory the amount of sedimentary rock on the crust must
have been much less than at present. Finally, we may
carry our imagination back to a time when the physical
conditions were very different to those now existing, when
very little stratified rock had been formed, and when most
of the rock at the surface was such as had consolidated
from a state of igneous fusion. *
From this point of view we may regard the Igneous
rocks as parts of the original magma from which the crust
was formed, and all other as having been ultimately derived
from portions of that magma ; so that all rocks may be
classed as either Original or Derivative.
Furthermore, it is obvious that in the lapse of time the
structures and component minerals of the members of both
these great divisions must be more or less changed by the
CHAP. II.] CLASSIFICATION OF ROCKS.
331
influence of the various agencies to which they have been
subjected ; especially if in the course of their history they
have been covered up by thick layers of superincumbent
rock, and exposed while damp to a higher temperature and
to a great pressure. These are the Altered or Metamorphic
rocks already mentioned, and it is important to note that
these rocks may once have belonged to either of the other
great groups, i.e., they may have been either Original or
Derivative.
A comparison of the Tectonic and Genetic methods of
classification will show that the two arrangements are
parallel. The two primary rock-groups in each correspond
exactly with one another, for all the Original or Igneous
rocks are unstratified, and all the Derivative rocks are-
stratified. Both lines of thought also lead to the establish¬
ment of subdivisions to include the Metamorphic rocks,
or those which are altered from their primary condition.
From the coincidence of these results we may feel con¬
vinced that the most natural and appropriate classifica¬
tion of rocks is into the following groups and sub¬
groups :—
Primary groups. Sub-groups.
1. Original, Unstratified or f A. Unaltered.
Igneous Rocks. ( B. Altered.
2. Derivative, Stratified or ( A. Unaltered
Aqueous Rocks. ( B. Altered.
1. Igneous Rocks strictly comprise only those whicb
have consolidated and crystallized from a state of igneous
fusion, and none of them present that regular kind of
stratification which is characteristic of all rocks deposited
in water. They occasionally occur in the form of sheets
which are interbedded with truly stratified rocks, and thus
simulate strata, but these are either the outspread termi¬
nations of very liquid lava-streams, or are intrusive sheets
of molten lava which have been injected between two strati¬
fied beds.
Igneous rocks are, in fact, either parts of the deep-
seated masses of molten material which lay beneath the
roots of ancient volcanoes, and were intruded upwards
into the rocks through which the volcanic orifices were
:332
STRUCTURAL GEOLOGY. [PAET II.
opened, or they are portions of the lavas which were
actually ejected through these orifices on to the surface of
the earth. They are always, therefore, intrusive or else
eruptive rocks.
2. Derivative or Stratified Rocks may be frag¬
mentai or crystalline ; those which have been mechani¬
cally formed are all fragmental ; those which have been
•chemically precipitated are generally crystalline, and those
composed of organic remains are sometimes partially crys¬
talline. These deposits are often called the Aqueous or
Sedimentary rocks, but if we use these terms strictly we
should exclude those accumulations which have been
formed by the agency of the wind, and which are by some
formed into a fourth class under the name of Aerial or
Eolian rocks. These are, however, of comparatively small
importance, and need not be excluded from this class if we
.adhere to the names of Stratified or Derivative rocks. In
the same class also should be placed those volcanic ¡ejecta-
menta which sometimes fall on land and sometimes into
the sea, and which are generally more or less stratified.
These, however, are sometimes considered as belonging to
the Igneous rocks, as resulting from volcanic explosions,
though they have not cooled in place from a state of fusion.
Mechanical deposits are therefore divisible into three sec¬
tions: 1, Aqueous; 2, Eolian; 3, Volcanic.
Metamorphic Rocks.—Although these are naturally
included under one or other of the primary rock-groups,
yet it is often convenient to regard them as forming a
distinct group, comprising all rocks which have been so
altered in course of time, that either their structure or
their mineral composition, or both, are different from that
which they first possessed. The characters exhibited by
some appear to have been mainly produced by the mecha¬
nical stress of vertical or lateral pressure ; others have been
affected by the chemical action of percolating waters and per¬
meating gases at depths where these influences are assisted
by that of heat ; so that their ingredients have formed new
combinations, and have re-arranged themselves in crystal¬
line forms. All these causes have combined to produce a
number of rocks which are more or less altered, and are
therefore different from any of the deposits that are now
CHAP. II.J CLASSIFICATION OF KOCKS.
333;
being formed on or near the surface of the earth. The-
■whole group falls naturally into two sections, viz., the Meta-
morphic igneous rocks, and the Metamorphic derivative
rocks.
Determination of Rocks.—The general relations of
rock-masses must be studied in the open country by
examining the structure of the rocks which are exposed in
pits, quarries, and cuttings of all kinds. It is in this way
only that the facts of geology are obtained, and by such«
observations the student will easily learn to determine at
once whether any given rock be Igneous, Stratified, or
Metamorphic. Mr. Jukes has well remarked that " a
blind man might just as well try to learn the art of paint¬
ing, as any one attempt to understand geology without
personal observation of those objects on the structure of
which the whole science depends."
It would be useless, therefore, to burden the reader's
memory with minute directions for distinguishing between
hand specimens of the three great classes of rocks above
mentioned ; but the case is different when the necessity
arises for distinguishing between the different members of
any one class of rocks.
The naked eye, or a hand-lens, will generally inform us
whether a rock be crystalline or not, and will give us some
insight into the textural arrangement of its component
particles ; a knife will often guide us in ascertaining what
minerals it is composed of by the degree of hardness and
resistance they possess.
It is very unsafe, however, to trust in such rough and
ready methods of determination, especially in the case of the-
igneous rocks, and geologists were formerly often led astray
by omitting to avail themselves of the more accurate means
of determination. These other methods are miscroscopio
and chemical analysis.
Microscopic Analysis.—It is now universally admitted
that the microscope is of the greatest use in determining
the mineral composition, texture, and probable method of
origin of all kinds of rocks, and that -without its assistance-
it would be and has been impossible to arrive at a natural
classification of the igneous rocks.
For the purpose of examining a rock under the micro-
534
STRUCTURAL GEOLOGY.
[PAET II.
scope, it is necessary to prepare a thin slice of it. If the
rock is a hard siliceous one, it must be sliced by a cutting
machine, or a thin chip of it may be taken, ground smooth,
and polished on one side. The polished surface is then
cemented with Canada balsam on to a piece of plate glass,
and the other side is ground down until the section is of
the required thinness and transparency. The Canada
balsam is then melted, and the slice is carefully pushed off
into a dish of turpentine, in which it is washed, so as to
remove all traces of the emery powder and other sub¬
stances used in the grinding and polishing process. The
preparation is then mounted on a glass slide with Canada
balsam, and overlaid with a thin glass cover, care being
taken to remove all air-bubbles from inside.
In the case of softer rocks a slice may be cut with a
small saw, one side polished on a sharpening stone (Water
of Ayr stone), and cemented on to a slide ; the other side
may then be rubbed down in water till thin, polished as
before, and the dust removed by a brush. It is then
covered with a glass slip.
The advantages accruing from the microscopical exami¬
nation of such slices are especially great in the case of rocks
whose texture is so fine and close that their mineral com¬
position could not be ascertained by examination with
ordinary hand-lenses. As Dr. Geikie observes : " A rock-
section prepared in this way enables us to ascertain with
precision the manner in which the different minerals are
built into each other, and often throws a flood of light on
the origin of a rock and on the subsequent changes which
the rock has undergone. It furnishes an opportunity of
•applying the delicate analysis of polarized light, and thus
reveals points of structure in the composition of a rock
which could not be ascertained in any other way."
For information regarding the methods of examining
rock-slices imder the microscope, the reader is referred to
Professor Cole's " Aids in Practical Geology," and Eutley's
Study of Rocks."
Chemical Analysis.—Though in the case of crystalline and
fragmental rocks a microscopical examination will usually
enable us to decide what minerals are present, yet it will not
do so in the case of certain igneous rocks (Volcanic glasses).
CHAP. II.] CLASSIFICATION OF ROCKS.
335
neither will it reveal the actual proportions of the different
minerals or elements present in stratified rocks. It is only
by chemical analysis that we can ascertain the ultimate
chemical composition of such rocks, and the relative pro¬
portion of the different elements they contain. It is in all
cases desirable to ascertain the chemical composition of a
rock, for it often reveals the presence of minute quantities
of certain minerals or elements which cannot be discovered
by the miscroscope : and in the case of a glassy igneous
rock we can compare the analysis with that of allied crys¬
talline varieties, the mineral constituents of which are
already known.
CHAPTEE III.
original or igneous rocks.
A. Unaltered.
\ S these have solidified from a state of fusion by heat,
their present appearance will obidously be the result
of two causes : 1, their chemical constitution ; 2, the cir¬
cumstances of cooling. To the former is due the presence
of certain minerals and the absence of others ; on the latter
depends the structural condition of the rock, whether it is
a glass, or has assumed a more or less crystalline structure.
There are in fact five points of difference among Igneous
rocks, which are of special importance ; these are :—
1. Mode of occurrence as rocks,
2. Internal structure.
3. Chemical composition.
4. Mineral composition.
5. Specific gravity.
Each of these characters might be made the basis of a
classification.
1. Mode of Occurrence.—Viewed as rock-masses, they
occur in various positions and exhibit different forms,
according to the circumstances under which they have
solidified. These differences wiU be described in a future
chapter; but as it was formerly customary to classify
Igneous rocks according to such differences, they may be
briefly referred to here. The divisions made on this basis
were : (1) Volcanic, or those which had cooled down in
the open air ; (2) Trappean, or those which had solidified
under comparatively slight pressure, as on the bed of the
chap. iii.] original or igneous rocks.
337
ocean ; and (3) Plutonic, or those which had solidified
deep within the earth. The name given to the second
group was from a Swedish word, trappa, a stair, because the
ends of the tabular rock-masses referred to this division
had often a step-like aspect. More careful investigation,
however, showed that many of the more typical members
of this division had solidified in the open air, and that it
was impossible to fix on any set of characters which
separated them from the ordinary volcanic rocks or lavas,
as they are often called. It then became usual to divide
the igneous rocks simply into Volcanic and Plutonic,
but here it at once became difficult, owing to the more
ready crystallization of the basic kinds, to know where to
draw the line. Hence these terms have been abandoned
for purposes of precise classification, and are only used as
general inclusive names of no very definite meaning, which,
however, are often useful.
2. Structure.—If the student will examine any masses
of igneous rock in situ, or any collection of rock specimens,
he will soon perceive that they exhibit great differences of
texture, some being glassy, some compact and stony, others
plainly crystalline.
We know also, as a matter of experiment, that from the
same materials, after fusion, may be obtained either a
translucent glass or an opaque stone, built up more or less
completely of crystalline particles, and by means of the
microscope it is found that the matrix, or ground mass, of
a rock may be in one of the following conditions :—
a. It may be a true glass (when it is often termed the
hase). In this case, when a thin slice of it is placed be¬
tween the crossed nicol prisms of a microscope with
a polarizing apparatus, the field remains dark as the
stage is rotated. Rocks with a glassy base are said to
be vitreous.
b. If small, faint, and rather ill-defined patches of light,
like the ghosts of crystals, appear and disappear as the
stage is rotated, so that no part remains dark throughout
a whole revolution, the ground mass is said to be crypto-
crystalline.
0. If the patches are more definite in shape and rather
larger in size, sometimes showing bright tints, so that it is
z
338
STRUCTURAL GEOLOGY.
[PAET II.
possible to identify the different component minerals, the
mass is said to be micro-crystalline. In this state, however,
the crystalline grains are often a little indefinite at the
boundaries, and do not commonly exhibit the outline of
their normal form.
D. If the slide is a mass of well-defined crystals, clearly
distinguishable one from another, of which those first
solidified generally give indications of their normal external
form, the rock is then said to be crystalline.
The second of these conditions is in many cases the re¬
sult of devitrification, the gradual change from a vitreous
to a semi-crystalline state which takes place in a glass under
certain conditions. In such case a crypto-crystalline rock
might fairly be regarded as metamorphosed ; but seeing that
we are not yet able to distinguish satifactorily between cases
where the structure is original and where it is secondary,
the separation is not generally attempted. Neither is it
easy at present to separate rocks with this structure from
those of the micro-crystalline group, so that we shall speak
of both inclusively as semi-crystalline.
Glassy and semi-crystalline varieties are of frequent
occurrence among the acid rocks, but are rare and more
limited in mass among the basic rocks, and in some
instances have not yet received distinctive names.
Ophitic Structure.—This is caused by the occurrence of
large crystals of one mineral which include a number
of smaller and previously formed crystals of another
mineral. Thus in many rocks consisting of felspar and
augite, the former has crystallized out before the latter, so
that the augite seems to form a ground mass or matrix in
which the felspar crystals are embedded. Such rocks
were formerly called Ophites, but the same kind of struc¬
ture has since been found in many other rocks, so that the
name is only used as a descriptive one. Such rocks
often present the appearance known as lustre-mottling, the
small crystals interfering with the play of the light from
the broken surfaces of the large crystals as the specimen is
turned in the hand.
Porphyritic Structure.—Conspicuous isolated crystals of
any one or more of the component minerals may occur in
glassy, semi-crystalline, or crystalline rocks, giving what is
CHAP. III.J ORIGINAL OR IGNEOUS ROCKS.
339
termed a porphyritie structure. The name Porphyry was
first applied to the dark red or purple rock, speckled with
light-coloured felspar, obtained from Jebel Dokhan in Egypt,
and largely used by the Romans for decorative purposes.
By some means the term became applied in process of
time to rocks of the most diverse mineral composition, but
exhibiting the same peculiarity of structure, viz., the con¬
spicuous occurrence of one mineral in a more compact
matrix. The adjective porphyritie is now always used with
this meaning, which of course the etymology {porphyra,
purple) does not sanction, and the name Porphyrite has
been applied to a definite kind of rock ; but the term
Porphyry is seldom used as a designation in England.
Crystalline rocks present several textural varieties ; they
may be coarsely crystalline, like many Granites ; they may
be finely crystalline and even grained (granulitic) ; they
may be porphyritie; and finally, two of the component
minerals may be so intermixed and interwoven with one
another by simultaneous crystallization as to produce
a curious and easily recognized pattern. This is the
structure of Pegmatite or Graphic Granite and hence
it is termed pegmatitic or graphic. Sometimes the matrix
of a porphyritie rock has this structure on a minute scale,
which is often called micro-pegmatitic, but the term micro-
graphic, recently proposed by Professor Bonney, is pre¬
ferable. A rock with a glassy base is said to be microlithic
when it is crowded with minute crystals or microliths.
If the mass of a rock is full of small cavities or vesicles
it is said to be vesicular ; the cavities are often elongated
in one direction, and when they are subsequently filled up
with some mineral, such as quartz or calcite, the infiltra¬
tion is called an amygdule or amygdaloid (amygdalon, an
almondl, and the rock is said to be amygdaloidal.
3. Cnemical Composition.—All the minerals which
ordinarily enter into the composition of igneous rocks are
siUcates, and free silica in the form of quartz is also fre¬
quently present. The total percentage of silica which they
possess is sometimes as high as 80, sometimes as low as 40,
or a little less ; hence these rocks are often simply divided
into acid and hasic, seconding as they contain more or less
than 60 per cent, of this mineral.
340
STRUCTURAL GEOLOGY. [PAET II.
So marked, and, within certain limits, so constant are
these differences all over the world, that Bunsen was led to
suppose they might be due to the existence of two distinct
magmas or liquid layers of different chemical composition
beneath the crust of the earth, the intermediate kinds of
rock resulting from a mixture of these two magmas in
varying proportions. Duxocher subsequently extended and
elaborated this theory, which may be called Bunsen's law,
but connected it with certain physical speculations which
are not generally accepted.
Later investigations have shown that different portions of
the same rock-mass, and in the case of certain rocks even
parts of the same hand specimen, often differ considerably
in chemical composition. This is especially the case with
porphyritic rocks which have a glassy or crypto-crystalline
base ; this base may be regarded as the mother liquor
which remained after the separation of the crystals from
the original magma, and analyses show that it is invariably
more acidic than the portion which has crystallized.'
There is, moreover, a difference, not only in the proportion
of süica, but in that of the alkalies, potash predominating
in the glassy base, and soda in the crystalline portion.
Commenting on these facts, Mr. Teall remarks : " If after
crystallization has progressed to a certain extent in a magma
of andesitic [i.e., intermediate] composition, we separate
the crystals from the part remaining liquid, the former
taken together will have the composition of a basic rock,
the latter of an acid rock. It is scarcely possible that this
connection can be the result of chance, and if not, it seems
to suggest that those variations in the composition of
igneous rocks which are expressed by Bunsen's law may be
due to differentiation produced in an originally homo¬
geneous mass in consequence of progressive crystallization."
(Op. cit., p. 45.) In other words, it suggests that the sub¬
terranean magma is homogeneous, and that separation into
portions of different chemical composition is effected during
its upward passage through the crust.
4. Mineral Composition.—The chief mineral compo¬
nents of igneous rocks have already been described, but it is
^ See Teall's " British Petrography," p. 42.
CHAP. III.J OKIGIXAL OR IGNEOUS ROCKS.
341
seldom that many of these occur together in one rock, and
certain laws or rules can be laid down regarding their
paragenesis or association. We can easily understand
that when the original molten mass contained a large per¬
centage of silica, its crystallization would give rise to the
formation of the more highly silicated felspars, and there
might still be a surplus of silica which would remain
as free quartz ; while in a mass with a low percentage of
silica, there might only be sufficient to form the less highly
silicated felspars, without any surplus remaining. As a rule,
therefore, free quartz is associated vrith the more highly
silicated felspars, such as Orthoclase, Albite, and Oligoclase ;
quartz may occur as an accessory mineral in company with
Labradorite or Anorthite, but never as an important con¬
stituent of the rock.
Hornblende also is more commonly found in association
with the highly silicated felspars, while Augite is generally
associated with the more basic felspars, and with Leucite
and Nepheline.^ Olivine, also, as might be expected, is
always associated with the more basic felspars, and some¬
times occurs alone with Augite and Enstatite. Hyper-
sthene, a mineral less common than is usually supposed,
has the same habit, but is sometimes found with the more
highly silicated felspars.
5. Specific Gravity.—The specific gravity of a rock
depends partly on its crystalline ingredients and partly on
the presence or absence of a glassy base. The specific
gravity of a mineral in a glassy condition is always less than
that of the same substance in a crystalline condition. Thus,
quartz crystal has a specific gravity of 2-6, but if fused
under the oxyhydrogen blow-pipe it solidifies into a glass
with a specific gravity of only 2'2, which is that of the
natural silica glasses. Opal and Hyalite. Mr. Teall re¬
marks : " The principal interest which attaches to specific
gravity, so far as Igneous rocks are concerned, lies in the
fact that it stands in close relationship to their chemical and
physical properties. If we compare rocks in the same
physical condition, then the specific gravity is seen to vary
' But Hornblende and Augite sometimes occur together in the
same rock, and Augite is sometimes altered into Hornblende.
342
STRUCTURAL GEOLOGY. [PAKT II.
with the chemical composition in a tolerably definite
manner If, on the other hand, we compare rocks of
the same composition, then the specific gravity is seen to
depend on the physical condition." (Op. cit., p. 48.) His
tabulated statement shows that,
1. Specific gravity increases as the silica percentage
decreases.
2. Specific gravity decreases as the amount of glass in a
rock increases.
Classification.—The systematic classification of ig¬
neous rocks is a matter of great difficulty, and all the arrange¬
ments which have been proposed are confessedly more or
less artificial groupings. Eocks must, of course, be named
before they can be classified, and though at first sight it
seems easy to define rocks according to the minerals of
which they consist, in practice it is not easy, for so vari¬
able are the rocks in this respect, that it is difficult to
say what minerals may, or may not, occur in one kind of
rock. A mineral has a definite form and composition, but
a rock is only an aggregate of mineral particles, and though
we may give a name to an aggregate of certain minerals,
we must be prepared for any amount of variety in their
relative proportions, and for the presence of particles
of other minerals besides those by which the rock is
named.
The only glimpse of a natural order among igneous rocks
is that afforded by Bunsen's law, and by what Mr. Teall
terms the order of progressive consolidation, that is, the de¬
velopment of different magmas out of one original liquid
mass by progressive stages of cooling and crystallization.
From this point of view it is clear that chemical composi¬
tion and specific gravity are points of primary importance,
and that mineral differences are only important so far as
they indicate difference of chemical composition.
Since, however, the nomenclature used is founded on the
presence or absence of certain minerals, it must be possible
to classify rocks according to their mineral differences, and
such a clasification must make it easier for the student to
remember these differences. At the same time the arrange¬
ment must be based on such a selection of minerals as will
best express the variations of chemical composition, and it
CHAP. III.] ORIGINAL OR IGNEOUS ROCKS.
343
should also accord with the results of observation in the
field with respect to the association of rocks with one
another.
Finally, attention must be paid to the relative quantities
of the different constituent minerals, for the mere presence
of quartz grains, or of highly sihcated felspars, is not a
proof that the rock is a very acid one ; that will depend on
the quantity present : again, two rocks may have the same
mineral ingredients, and yet have a different chemical
composition.
In framing the following arrangement of the rocks the
author has kept these points in mind, and, so far as the
existent nomenclature allows, he has endeavoured to express
the recognized scale of chemical composition (from acid to
basic) in terms of the mineral constituents. The order of
the rock-groups is the same as that given by Professor
Bonney in the first edition, but these are now gathered
into classes which are defined in such a manner that the
reader may at once see on what principles the groups are
established.
The subdivisions are made to depend on structural
characters, and the terms Granitoid and Lithoid are adopted
to express these : the first including coarsely granular,
granulitic, and graphic and micrographie varieties ; the
second microcrystalline, micrographie, and microlithic
varieties. It must be remembered that there are no hard
and fast lines between the groups, each group being con¬
nected with the others by rocks of an intermediate
character.
The minerals mentioned are those which are regarded as
essential to the definition of the class. The felspar may be
either orthoclase or plagioclase, or it may be partially re¬
placed by nepheline or leucite. The hornblende may, or
may not, be associated with augite or mica. These minor
differences form subsidiary groups, which will be men¬
tioned when each class is treated in detail.
The glassy rocks are not included in this tabular view,
but each of the groups may pass into a volcanic glass when
the material has cooled rapidly.
344
STRUCTURAL GEOLOGY.
[paet ii.
Classification of Igneous Bocks.
Class I. Acid Rocks. Percentage of Silica, 66 to 76.
Sp. Grav., 2*5 to 2'7. Dominant minerals, Quartz and
Felspar. Some mica, hornblende, and augite present.
Granitoid type. Lithoid type.
Group A. Granite. Quartz, Felsite, and Ehyo-
lite.
Group B. Tonalité. Quartz-Porphyrite and Da-
cite.
Class II. Sub-Acid Rocks. Percentage of Silica, 55 to
65. Sp. Gray., 2 7 to 2'8. Dominant minerals. Felspar
and Hornblende ; Quartz present in small grains.
Granitoid type. Lithoid type.
Group C. Syenite. Felsite and Trachyte.
Group D. Diorite. Porphyrite and Andésite.
Class III. Basic Rocks. Percentage of Silica, 40 to
55. Sp. Gray., 2'9 to 3'2. Dominant minerals. Plagio-
clase felspar, Augite, and Oliyine ; no Quartz.
Group E. Gabbro, Dolerite, and Basalt.
Group F. The Peridotites and Picrites.
Geoup a.
I. Granite is a crystalHne compound of quartz, ortho-
clase, and mica, the particles of all three being generally
yisible to the eye. The orthoclase is sometimes replaced by
microcline, as in parts of the granite of Leinster ; and oligo-
clase and albite are often associated with it. The mica may
be muscoyite or biotite. When hornblende is present the
rock is called Hornblendic granite ; if augite takes its place
we haye Augite granite. When mica is scarce, and the
quartz and felspar are so intergrown that the form has a
rude resemblance to Hebrew characters, it is called Graphic
granite, or Pegmatite.
Eurite, or Micro-granite, is a fine-grained rock with only a
little mica, and is often found in the yeins which proceed
from a mass of granite. It often contains porphyritic
quartz or felspar. The matrix is sometimes micrographie.
chap. iii.] original or igneous rocks.
345
2. Quartz-felsite has a micro or crypto-crystalline
matrix of quartz and orthoclase, with porphyritic crystals of
quartz and felspar. The rock has also been called Elvanite,
but this is not a good name, for the word is only a local
Cornish term for a dyke, and many of the elvans do not
consist of Quartz-felsite. Many of the crypto-crystalline
felsites are only devitrified Liparites.
3. Liparite, or Quartz-trachyte.—This is the Vol¬
canic form of a rock which, if entirely crystalline, would
be a granite. The ground mass consists mainly of
minute crystals of sanidine embedded in a glassy base,
with porphyritic crystals, or quartz, or sanidine. Soda-
felspar, hornblende, augite, and mica are sometimes present.
The rock is often called Bhyolite, but it would be con-
Tenient to restrict the latter of these names to the more
compact varieties, which often give indications of a fluidal
structure.
Geoup B.
The members of this group resemble those of Group A,
but have a Soda-felspar (Oligoclase, Albite, or Andesine)
instead of Orthoclase. Hornblende is generally present,
and. Quartz always in fair quantity.
1. Tonalité is the Granitic form. It takes its name
from the Tonale Pass in South Tyrol, near which is a large
mass of it : this contains quartz, oligoclase, mica, and
hornblende.
2. Quartz-porphyrite has the same structure as
Quartz-felsite, the felspar only being different, and as
many of them exhibit that peculiar felsitic structure which
is so generally the result of devitrification, these are pro¬
bably only altered Dacites.
3. Dacite answers to Liparite, the base being glassy
and the minerals occurring porphyritically. It receives
its name from the ancient Dacia, in which district it
occurs. It is also called Quartz-Andesite, from the Andes,
where it is also found. These rocks are analogous to the
quartz-trachyte of the former group. There are also very
glassy forms, but these as a rule can only be distinguished
by chemical analysis from Obsidian and Pitchstone, and
have not yet received distinctive names.
346
STRUCTURAL GEOLOGY.
[paet ii.
Obsidian and Pitchstone are volcanic glasses, formed
by the rapid cooling of Liparites and Dacites. There is no
optical difference between the more acid and less acid varie¬
ties, which can only be distinguished by chemical analysis.
They both consist chiefly of glass, but contain scattered
microlites and occasional crystals. They differ from one
another thus : Obsidian has a very conchoidal fracture and
a glassy lustre (being very like dark glass) ; Pitchstone has
a more splintery fracture and a resinous lustre. It is
not certain to what this fairly persistent difference is due,
but probably to the relative abundance of microbths.
G-eoup C.
1. Syenite.—This rock is usually defined as consisting
of orthoclase and hornblende, but it is doubtful whether
such a rock exists, and it is at any rate very rare. More¬
over a rock of this composition would not be the plutonio
representative of Trachyte, which has from 60 to 64 per
cent, of Silica. It is necessary to admit the presence of
quartz as an essential constituent, though it is only visible
by the aid of the microscope. The name is derived from
Syene in Egypt, but the rock there is said to contain visible
crystals of quartz, and is therefore a Hornblendic granite.
Plagioclase is very often present, and mica frequently.
Micrographic Syenite is a rock with a micrographie matrix
of quartz and orthoclase, enclosing porphyritic crystals of
hornblende and sometimes of oligoclase also.
2. Ortho-felsite is a rock with afelsitic or semi-crystal¬
line matrix, with porphyritic crystals of orthoclase and
hornblende.^
8. Trachyte has the same structure as Liparite, but is
without the porphyritic crystals of quartz, the silica being
entirely in the glassy base.
Sttb-Geoup C k—A parallel series is formed by rocks in
which the felspar is partly replaced by nepheline. Thus
^ It must be remembered that many Felsites are only devitrified
obsidians, and that some petrologists consider all rocks with a
felsitic matrix to be devitrified. A name is wanted for compact
crystalline Syenites.
chap, iii.] original or igneous rocks.
347
NepJieline Syenite, or Foyaite (from the Foya Mountains in
Portugal), consists of Orthoclase, Nepheline, Hornblende,
and a little Quartz. Phonolite consists of the same minerals,
but the quartz is not visible, and the structure is trachytic :
plagioclase felspar, leucite, nosean, and mica often occur
as accessories, or even in sufficient abundance as to con¬
stitute well-marked varieties ; augite also may occur. The
semi-crystalline and glassy varieties have not received
separate names.
Sub-Geoup C —This forms a portion of the rather
limited series of rocks often distinguished as Mica-traps.
Mica (more commonly magnesia-mica) is always a con¬
spicuous constituent. Some free quartz is occasionally
present, and more or less hornblende or augite. The
nomenclature of the group is imperfect. It will probably
be found convenient to restrict the term Minette to the
crystalhne form, and designate the others respectively
Mica-felsite and Mica-trachyte.
Geoup D.
The members of this group only differ fi,-om those of
Group B in the relative quantity and size of the included
quartz grains.
1. Diorite.—As in the case of Syenite, this rock has
been regarded in its typical form as a quartzless rock
consisting of plagioclase and hornblende. Such rocks do
occur, but they cannot be considered as the plutonio equi¬
valents of Andésites, while the majority of the rocks which
have been called Diorites contain quartz, though it is not
discernible without the aid of the microscope. Mr. Teall's
view of the constitution of Diorite is much more logical
and satisfactory. He regards quartz as an 'essential con¬
stituent, and therefore excludes the more basic varieties ;
as he observes, " in the Andésites the silica is present in
the interstitial matter, in the holocrystalline rock it sepa¬
rates out as quartz " (" British Petrography "). In many
of the plagioclase-homblende rocks the hornblende is a
paramorph after augite (see p. 823).
The felspar is generally oligoclase or andesine, but may
348
STRUCTURAL GEOLOGY.
[PAET II.
in some cases be labradorite ; the rock-structure may be
granular or micrographie. If augite be present we have
an Augite-Diorite, which passes into Gabbro by a diminu¬
tion of the quartz. The presence of mica gives a Mica-
Diorite. The name Aphanite is applied to a fine-grained
rock in which the components can only be seen with a
microscope.
2 and 3. Porphyrite and Andésite differ from Quartz-
porphyrite and Dacite simply in the absence of free por-
phyritic quartz, but the Porphyrites are now regarded as
only devitrified Andésites.
Stjb-Geoup D\—Plagioclase-Nepheline rocks are
not common, and do not occur in Britain. Tephrite is an
Andésite in which NepheHne and Leucite are present ; and
Theralite is a coarse-grained rock of the same composition.
They may be called Nepheline-Diorite and NepheHne-
Andesite.
Sub-Group D —This contains the remainder of the
Mica-trap group, which differs from the Minette group as
•above described in that a plagioclastic felspar replaces the
orthoclase. There is at present the same deficiency in the
nomenclature, the term Kersantite being applied to the
crystalline or more conspicuous representatives.
Group E.
The nomenclature of this group, though four members
are common, is at present not very precise. The type-
rock maybe defined as consisting of a plagioclastic felspar,
generally labradorite, and of augite, with more or less iron
peroxide, and commonly some olivine.^ Magnesia mica is
often present, and sometimes rather abundant. Gabbro is
a coarsely crystalline rock of granitic structure only occur¬
ring in masses, dykes, and veins. In it diallage frequently
takes the place of augite, and the name Norite is given
to a hypersthene-felspar rock. A variety with bttle augite
' As this mineral is frequently abundant enough to become an
important constituent, while in other cases it is entirely absent,
some have proposed to restrict the term dolerite to the former ; but
the distinction can hardly be maintained.
CHAP. Ill.j ORIGINAL OR IGNEOUS ROCKS.
34?
and mucli olivine is known as Troctolite. A rock in wHch
hornblende wholly or partially replaces the augite is called
Hornblende-Gahbro. When the two principal constituents
are visible to the unaided eye, forming a speckled whitish
and black rock, it is called Dolerite, and when the con¬
stituents are so minute that the rock is black, it is called
Basalt.' No name is thus applied to the (rare) semi-
crystalline variety ; the glassy form, also rare and limited
in thickness (it not seldom exists as a mere coating to
ordinary basalt masses), is called Tachylite. This is a
black or brownish-black glass distinguishable from ob¬
sidian chiefly by its greater specific gravity and easier
fusibility with the blow-pipe.
Sub-Gtboup E —(a.) Nepheline-Holerite (or NepJielenite).
—The rocks of this differ from the last in containing
nepheline instead of felspar; there is the same want of
preciseness in the nomenclature. The coarser varieties are
easily recognized; the finer require microscopic examina¬
tion. The rock, however, is commonly a little lighter in
colour than a felspar basalt.
(6.) Leucite-Dolerite (or Leucitite).—The rocks of this
differ from the felspar-basalts in containing leucite instead
of felspar. The rock, as a rule, is decidedly lighter in
colour than the others, and the augite is sometimes green.
Intermediate forms, containing two or all of the three
minerals felspar, nepheline, and leucite, occur.
Gtroup P.
The nomenclature of this rather rare group is also in
need of revision. The great majority of its members are
crystalline, and the rock appears to be generally a deep-
seated one. Olivine is the essential constituent, and with
it enstatite or augite, or both together, are commonly asso¬
ciated. Hornblende occurs occasionally. There is always-
some iron peroxide, and small quantities of chrome and
nickel are generally detected by analysis. Lherzolite ^ is
' The term Anamesite has been applied to intermediate varieties.
' From Lake Lherz in the Pyrenees (Ariège), where the rock
occurs.
350 STRUCTURAL GEOLOGY. [PAET II.
the name given to a species consisting of olivine, enstatite,
augite (variety diopside) and picotite (a variety of spinel).
Dunite 1 appears to be chiefly olivine and chromic iron.
Picrite contains olivine, augite (or hornblende), with
usually some mica and often a little felspar. In Picrite
the pyroxenic constituent generally rather predominates
over the peridotic, and the rock appears to be rather an
ancient one. It may be regarded as intermediate between
Dolerite and a Peridotite.'' Here also may be placed
Eulysite, in which garnets are always conspicuous.
The following are names and terms which are often used,
and of which some explanation seems necessary :—Felstone
and Greenstone designate a large group of rocks (most of
which would anciently have been included in the Traps)
which are too compact in their structure to allow their
crystalhne condition and mineral constitution to be readily
distinguished,—felstone including the eurites, felsites, por-
phyrites, and, perhaps accidentally, some phonolites, and
yreenstone the more compact diorites, aphanites, diabases,
and slightly decomposed basalts. The term Greystone has
been occasionally used for some of the darker felsites,
the phonolites, and some of the paler basalts.
Melaphyre is another vague term of dubious advantage.
It has been applied to dark compact rocks of considerable
geologic age, defined as normally consisting of " felspar
(plagioclase), augite, and iron peroxide." Further exami¬
nation has shown that some of these rocks are aphanites or
porphyrites, but the majority simply rather ancient basalts,
and so far as the term bears any precise meaning it is
equivalent to " probably basalt, certainly old." It has,
however, been used so vaguely, though with an air of
-exactness, that perhaps it is better not to employ it.
Fluid Cavities in the Crystals of Igneous Rocks.
—By careful observations on the minute structure of
^ From the Dun Mountain, New Zealand.
' The student wiU find more complete descriptions of igneous
rocks, with illustrations of their appearance imder the microscope,
in Professor GrenviUe Cole's " Aids in Practical Geologv " (Griffin
.and Co.), and in Dr. Hatch's " Petrology of Igneous Kocks " (Swan
Sonnenschein and Co.).
CHAP. III.] ORIGINAL OR IGNEOUS ROCKS.
351
crystals,^ Mr. H. C. Sorby bas shown that it is possible to
arrive at some interesting and remarkable conclusions as to
the liquid magma under which the crystalline particles of
granite and other igneous rocks were formed. The follow¬
ing is an abstract of his argument
When artificial crystals are examined with the micro¬
scope, it is seen that they have often caught up and en¬
closed within their solid substance portions of the material
surrounding them at the same time when they are being
formed. Thus, if they are produced by sublimation, small
portions of air or vapour are caught up, so as to form
apparently empty cavities ; or if they are deposited from
solution in water, small quantities of water are enclosed,
so as to form fluid-cavities. In a similar manner, if crystals
are formed from a state of igneous fusion, crystalhzdng out
from a fused-stone solvent, portions of this become en¬
closed, and on cooling remain in a glassy condition, or be¬
come stony, so as to produce what may be called glass- or
stone-cavities. From these facts he deduced the following
conclusions :—
1. That crystals containing only stone- or glass-cavities
were formed from a state of igneous fusion.
2. That crystals containing both water- and stone-
or glass-cavities were formed in molten rock under the
combined influence of highly heated water and great
pressure.
Applying these principles to the examination of igneous
rocks, Mr. Sorby proves from them the igneous origin of
aU, with this remarkable result, that the fluidity of the
more superficial lavas was a more purely igneous one than
that of the deeper seated trappean and plutonio rocks.
The minerals of erupted lavas contain stone- and glass-
cavities like the crystals formed in the slags of furnaces ;
but the blocks ejected from volcanoes contain water-
cavities as well, the amount of water indicating that they
were formed under great pressure and at a dull red heat,
when both liquid water and melted rock were present.
The crystals of the Cornish elvans and the Cornish and
Scotch granites contain both fluid- and stone-cavities, prov-
^ " Quart. Joum. Geol. See.," vol. xiv. p. 453.
352
STRUCTURAL GEOLOGY.
[PAKT II.
ing the presence of water, and perhaps also of gas, as well
as the conditions of great heat and pressure.
" On the whole, then," says Mr. Sorby, " the microsco¬
pical structure of the constituent minerals of granite is in
every respect analogous to that of those formed at great
depths and ejected from modern volcanoes, or that of the
quartz in the trachyte of Ponza, as though granite had
been formed under similar physical conditions, combining
at once both igneous fusion, aqueous solution, and gaseous
subhmation. The proof of the operation of water is quite
as strong as that of heat."
Subsequent investigation has, however, proved that liquid
enclosures may be of subsequent formation and due to the
partial solution of the crystal, and as it is difficult to dis¬
tinguish these from original enclosures, the application of
Mr. Sorby's results is not always easy.
Pyroclastic BocJcs.
The materials ejected from volcanoes being really vol¬
canic rocks may be mentioned here, though they have not
consolidated as rocks from a state of igneous fusion, but
consist of broken fragments of volcanic rocks ; hence they
are often called pyroclastic materials, from irvp, fire, and
icXaiTToe, broken.
Agglomerate.—This is a confused assemblage of an¬
gular blocks embedded in a paste of finer materials, and is
only found in close proximity to the volcano from which
all the fragments have been ejected. The blocks are
chiefly pieces of lava, but mingled with these are frequently
fragments of the local stratified rocks, which have been rent
by the outburst of the volcano. A volcanic agglomerate is
a terrestrial rock, but if the same materials are ejected into
lakes or seas, so that the blocks are more or less rounded
and water-worn, a volcanic conglomerate is produced.
Volcanic Ash, Tuff, or Peperino, are names applied
to the finer kinds of volcanic detritus ; the sand, dust, and
powder which is carried to a greater distance from the vol¬
cano. They are generally more or less regularly stratified,
whether the deposits have been formed on land or on sea-
bottoms, but in the latter case the volcanic ashes are
CHAP. III.J ORIGINAL OR IÜNEOU.S ROCKS.
353
mingled with the materials of ordinary stratified deposits,
and sometimes contain organic remains.
Tuffs may be subdivided into two classes, according to
the felspathic or pyroxenic nature of the base or ground
mass of the rock. The former are called Felstone Tuffs,
the latter Greenstone Tuffs.
Felstone Tuff varies in colour from white or grey to
yellowish-red or dark brown; in texture it is generally
earthy and flaky, and frequently contains broken crystals
of felspar, or small fragments of some trachytic rock.
Sometimes sand is mingled with this base, and the rock
passes into a felspathic sandstone. A nodular concretionary
structure is occasionally developed, the size of the nodules
varying from that of nuts to that of cannon-balls. The
more compact and indurated varieties are not easy to
distinguish from felspathic lavas, until the aid of the
microscope is called in ; this is the case with some of the
felspathic ashes or felstone tuffs of North Wales and the
Lake district.
Greenstone Tuff is usually dull and earthy in texture,
with a prevailing dirty-green or purplish colour. Coarser
gravelly varieties also exist, which pass gradually into
volcanic agglomerate. In the central valley of Scotland
there is a vast quantity of such tuff interstratified with the
ordinary sedimentary rocks ; Dr. A. Geikie describes it as
made up of a paste of abraded dolerite rocks, with fragments
of these rocks and of shale, sandstone, limestone, etc. It
has a prevalent green colour and is usually well stratified,
but a nodular or cannon-ball structure is not uncommon.
Similar greenstone tuffs occur in County Limerick, Ireland,
and some of these contain fragments of vesicular green¬
stone (? Dolerite), such as is not known anywhere in the
neighbourhood : these fragments are probably portions of
the upper scoriaceous surface of the lava which was then
rising in the ancient volcano from which the tuffs were
ejected (Jukes).
Pumiceous Sand.—Fine comminuted pumice is some¬
times ejected from volcanoes and distributed over large areas,
forming beds of light grey sand which may be interstratified
with volcanic ash and lava, or with aqueous sedimentary
deposits.
A A
354
STRUCTURAL GEOLOGY.
[PAET II.
Altered Igneous Bocks.
Those metamorphic rocks whieh have resulted from the
alteration of Igneous rocks will be described in a future
chapter, but a list of them may be given here :—
Gneissic Granite, Schorlaceous Granite, Zwitter-rock, and
Greissen, are altered granites.
Euphotide is altered Gabbro ; Diabase and some Horn¬
blende Schists are altered Dolerites and Diorites.
Serpentines are altered Peridotites.
Porphyroid and Schalstein are altered volcanic tuffs.
Palagonite is generally altered volcanic glass.
CHAPTEE rV.
DEEIVATIVE OR STRATIFIED ROCKS.
Their origin and Composition.—In Chapter III.
we haTe examined the igneous rocks which come from
the interior of the earth, and we have seen that they are
essentially siliceous, some of them, especially the granites,
containing crystals of pure silica, and all of them contain¬
ing silicate of alumina. Now pure sand consists of grains
of quartz, and pure clay consists of silicate of alumina
combined with water. It is obvious, therefore, that clay
and sand may be formed by the detrition and destruction
of an igneous rock, such as granite, and this process may
be actually watched wherever granite rocks are exposed to
the erosive action of water.
All the sandy (arenaceous) and all the clayey (argil¬
laceous) deposits, therefore, which are met with in the
earth's crust, have been derived from some previously
existing rocks, and these older rocks must either have been
igneous rocks, or must originally have been derived from
some igneous rock.
Similarly the siliceous and calcareous rocks of organic
origin have been formed out of the sUica and carbonate of
lime held in solution by the sea-water; these materials
having been primarily derived from the decomposition of
the soluble silicates contained in igneous rocks.
The varieties of rock produced by the intermixture of
the aluminous, quartzose, and calcareous elements are very
numerous. The proportions of these ingredients in any
deposit wiU depend partly on the character of the rocks
exposed to erosion in the area whence the chief supply is
derived, and partly on the amount of matter contributed
366
STRUCTURAL GEOLOGY.
[part II.
by organisms to the sediment which is accumulating
below. Thus the chemical composition of any stratified
rock is generally very variable : sands and sandstones
frequently contain fragments of other minerals besides
quartz ; clay almost always contains a certain amount of
lime and iron; and many limestones are rendered impure
by an admixture of clay or siliceous matter.
Classification and Description.—As already stated,
the whole group is naturally divided into two series—
A, the unaltered rocks ; B, the altered or metamorphic
rocks. In the present chapter we shall deal only with the
unaltered rocks, and it will be convenient to describe them
under the following heads :—
1. Arenaceous rocks.
2. Argillaceous rocks.
3. Calcareous rocks.
4. Ferruginous rocks.
5. Carbonaceous rocks.
1. Arenaceous Bocks.
Breccia is a mass of angular fragments, the interstices
between them being filled either by small deliris, sand and
dust, or by an infiltrated cement of carbonate of lime or
other mineral matter deposited by percolating water. The
agencies by which ordinary breccias are formed have been
described on p. 191; but Professor Bonney^ has called
attention to another class of breccias, which might be
mistaken for the former, but have really been formed in
a very different manner. These he terms " crush hreccias,"
and believes them to have resulted from pressure or strain
acting on a rock generally tmiform, but containing weaker
beds or portions, which have been crushed in situ into
fragments of all shapes and sizes. They occur principally
in the older rocks, and when the fragments are re-cemented
by infiltrated matter they closely resemble an ordinary
breccia.
Conglomerate is a mass of consobdated gravel or
shingle, the pebbles being roimded and water-worn, and set
' " Geo!. Mag.," Dec. 2, vo\ x. p. 435.
CHAP. IV.] DERIVATIVE OR STRATIFIED ROCKS. 357
in some kind of matrix whick binds them together into a
firm rock. The pebbles may consist of any kind of hard
rock or mineral which is capable of resisting continued
attrition ; and siliceous substances, from their great dura¬
bility, are naturally the commonest constituents. When
the rock consists chiefly of quartz pebbles it is called a
Quartz-conglomerate ; when the pebbles are limestone,
it is a Limestone-conglomerate, and so on. The matrix
enclosing the pebbles also varies in composition; some¬
times it is only sand, and the rock appears to have been
consolidated simply by pressure, so that the pebbles may
be removed by a slight blow; in other cases the matrix
consists of an infiltrated cement, either calcareous, ferru¬
ginous, or siliceous, and is sometimes so hard that a
blow produces a clean fracture through both matrix and
pebbles.
Sandstone is consolidated sand, and the particles are
usually quartz. Particles of flint or chert are rare, but
occur in some Tertiary sands. Sandstones vary greatly as
regards their degree of consolidation, from a mere sand-
rock which is just compact enough to stand with a vertical
face, to a hard and compact gritstone, which is capable of
being used as a millstone. The size of the grains also
varies from the minutest particles of quartz to grains as
large as peas, the rock then commencing to pass into a
conglomerate.
In the case of sand earned by water the particles are
buoyed up in the liquid, their relative weight being less
and their friction on the bottom and against one another
being proportionately diminished ; under such circum¬
stances a great amount of drifting and mechanical action
must be required to wear down any quartz fragments into
round grains. As a matter of fact, Mr. Sorby finds that
the sand brought down by rivers is often very little worn,
and that in sea-shore sands the proportion of well-worn
grains does not often exceed one-half.
But when drifted and worn by the wind in the open air
the amount of friction is necessarily very much greater,
and in sands so accumulated all the grains show signs of
attrition, most of them being completely rounded, and
their surface more or less roughened.
358
STEUCTUfiAL GEOLOGY, [PAET II.
Some of the sandstones associated with shallow-water
deposits are doubtless ancient blown sands formed on ter¬
restrial surfaces, and subsequently covered by strata of
aqueous origin. But Mr. A. R. Hunt has pointed out that
the sand of submarine shoals and sand-banks is kept
almost constantly in motion by the action of the waves,
and his examination of fine sand from the Skerries shoal in
the English Channel showed that a large proportion of the
grains were rounded.
There are many varietie.s of sandstone produced by the
admixture of other mineral substances with the quartz
grains.
Micaceous sandstone contains flakes of mica which are
sometimes so abundant as to cause the rock to split into
plates and slabs with glittering surfaces.
Felspathic sandstone contains grains of felspar, distin¬
guishable by their dull white, yellow, or reddish colour,
and peculiar fracture, imparting an earthy appearance to
the rock. When coarse and resulting from the direct
detrition of granitic or gneissic rocks, so that the grains of
quartz felspar and mica are angular and plainly visible, it
is called Arkose. Wben very fine, so that the constituents
cannot be distinguished without a microscope, it may be
called a sandy mudstone. When compacted by pressure
or by the infiltration of a siliceous cement, it is known as
Greywacke.
Calcareous sandstones.—Of these, three kinds may be
distinguished—1, Cornstone, a fine-grained rock, probably a
calcareous silt cemented by crystalline calcite ; 2, Calcareous
grit, a coarse-grained sandstone, the separate grains of
which are cemented together by crystalline calcite ; 3, Sand¬
stone, the component grains of which are embedded in
large crystals of calcite. These crystals form a kind of
mosaic, and each includes a portion of the original sub¬
stance of the rock.
Argillaceous sandstones, or consolidated silts, are always
fine-grained, and generally more or less laminated, so as
to split easily along the planes of bedding ; they are then
called Flagstones, and generally contain a proportion of
felspathic or micaceous material.
Glauconitic sandstone is one in which grains of glauconite
CHAP. IT.J DERIVATIVE OR STRATIFIED ROCKS. 359
are conspicuous, giving the whole a greenish tint. The
softer kinds are usually called Greensand. Some glauco-
nitic sandstones become calcareous grits by the infiltration
of a calcific cement. Gaize is a fine-grained micaceous and
glauconitic sandstone, occurring among the cretaceous
rocks of France and England.
Malmistone, as understood in England, is a siliceous rock,
but the silica is chiefly in a colloid state, and derived from
an organic source. It is a fine-grained whitish rock, which
looks like chalk at a little distance, and it is sometimes
calcareous from an admixture of fine chalky matter ; but a
pure malmstone is light and porous, microscopical exami¬
nation showing that it consists of very minute globules of
colloid silica with numerous broken sponge spicules. Its
porosity is due to minute cavities left by the solution of
many of the spicules. This rock is often called Firestone.
Siliceous sandstones.—Under this head may be placed
those sandstones which are compacted into hard rock by a
siliceous cement. They are distinguished from quartzite
by the fact that the cement is opal or chalcedony, while in
quartzite it consists of crystalline quartz formed in optical
continuity with the quartz of the original sand. Chalce¬
dony is the cement most frequently found, but opaline
cements are known, and an opaline sandstone containing
precious opal has recently been found in the interior of
Hew South Wales near Wincannia.
Peldon and Calliard are miners' terms applied to beds
of hard fine-grained siliceous stone, which have a smooth
and even fractiu-e.
Chert is a name applied to any siliceous nodule or layer
consisting of earthy or organic particles embedded in a
matrix which shows the radiating fibrous structure of
chalcedony. Flint differs in having a matrix which is
partly chalcedonic and partly microcrystalline. Both have
a glassy or flinty fracture.
2. Argillaceous Bocks.
Clay may be defined as a consolidated mud which con¬
tains sufficient hydrated silicate of alumina to be plastic,
or capable of being moulded. Kaolin, or porcelain clay, is
360
STRUCTURAI- GEOLOGY.
[PAET II.
nearly pure clay, white and free from iron ; its derivation
from the decomposition of granite has been described on
p. 197. It is now admitted, however, that the decay of
orthoclase resnlts in the formation of a hydrated mica as
well as of kaolin, both being probably produced simulta¬
neously—one by a total replacement, the other by a partial
replacement of the potash by water. Besides this secondary
mica, comminuted primary mica seems to be an essential
constituent of some clays, and they are generally coloured
by oxide or carbonate of iron, or by carbonaceous matter.
Some clays have, doubtless, been formed directly from
the waste of granite, gneiss, or other crystalline rock, but
many are only the washings of older stratified rocks. Mr.
W. M. Hutchings has lately made a careful examination
of certain coal-measm-e clays,' and finds that they have
resulted from the wear and decay of a granitic rock which
consisted of quartz, felspar, and two micas (muscovite and
biotite). Minute particles of all these minerals can be seen
in the clay under the microscope, but the fine material of
the ground mass seems to consist largely of a secondary
mica. He did not find anything which could safely be
regarded as kaolin, the most minute flakes which could
be separated by levigation consisting of mica, and he con¬
cludes that even the very finest powdery matter is mica¬
ceous. Other clays may be a mixture of kaolin with fine
micaceous material, but further investigation is required.
Pipe-clay is a pure white clay, resembling kaolin, but
liable to shrink when heated, and consequently not fit for
making china.
Siliceoiis clays.—Fire-clay contains a considerable quan¬
tity (often 40 or 50 per cent.) of fiine quartz-sand mixed
with the sihcate of alumina, and a good fire-clay must be free
from lime, alkalies, and iron, which act as fluxes ; it will
then stand a very intense heat without melting. The Stour¬
bridge fire-clay consists of about 64 per cent, of hydrated
sihcate of alumina, with 33 per cent, of fine sand and 2 per
cent, of iron-oxide. Tile and Pottery clays have a simila.r
composition, but generally contain a larger proportion of
' "Geol. Mag.," Dec. 3, vol. vii. pp. 264, 316, and vol. viii.
]). 164.
CHAP. IV.J DERIVATIVE OR STRATIFIED ROCKS. 361
iron. Fullers' earth is another siliceous clay, which has
the peculiar property of not being plastic in water, but
crumbles down into a fine powdery mud. The reason of
this is not clearly known. From a recent analysis by Mr,
Sanford, it appears to consist of an intimate mixture of
the silicates of alumina, iron, lime, and magnesia (50 to
55 per cent.), fine sand (30 to 35 per cent.), and combined
water (13 to 15 per cent.).
Gannister is a highly siliceous fire-clay, occurring in the
coal-measures, and largely used as a material for the
hearths of iron-furnaces.
Marl is a calcareous clay, that is to say, a clay which
contains a certain amount of carbonate of lime, and often
carbonate of magnesia. When the proportion of carbonates
is high, it becomes a chalk marl ; when low, it is a marly
clay. Strictly speaking, a marl should contain from 20 to
50 per cent, of calcareous matter; but many of the so-
called marls contain so little carbonate of lime that they
are really only marly clays.
The following are analyses of marls and marly clays :—
G-ault,
SuiTey,
a marly clay
(Sanford).
Keuper
Marl,
"Worcester
(Voelcker).
M arnés
irisées,
Lunéville
(Voelcker).
Upper
Gault,
Bedfordshire
(Severn).
Silica
46-43
53-62
43 •51
38 •21
Alumina ....
18-81
20-53
10^16
3 90
Carbonate of Lime
10-50
7 •69
18 92
53 50
Carbonate of Mag¬
nesia ....
■95
5^10
14 ^40
—
Magnesia as Sili¬
cate
•24
169
2^10
—
Oxides of Iron . .
9-97
4^79
5-75
2 00
Alkalies ....
•07
2^91
•88
Water of combina¬
tion
1048
4^45
4 24
2^00
97 •45
100^78
99 •96
99 •Ol
Shale is the general term for a laminated clay, often
called Bind or Blue-hind by miners and quarrymen; by
colliers a carbonaceous shale is called Batt.
362
STRUCTURAL GEOLOGY.
[PAET II.
Boulder clay is a clayey deposit, full of stones and boul¬
ders, generally unstratified, and due to tbe agency of ice.
3. Calcareous Bocks.
The processes by which calcareous rocks are formed have
been described in Part I., pp. 238 and 273. These rocks
may be roughly divided into two classes : (1) those which
are largely formed of organic debris—i.e., fragments of the
shells and skeletons of organic bodies ; and (2), those which
have been formed by the precipitation of carbonate of lime
from a state of aqueous solution. Those which come under
the first head are by far the most numerous and important,
but the proportion of recognizable organic fragments in
such rocks varies greatly, and the interstices between these
fragments are generally filled up by fine calcareous mud or
by a subsequent infiltration of calcite.
The fine powder or paste which forms the ground mass
of many limestones may itself be partly a chemical precipi¬
tate (see p. 277), hence it seems probable that nearly all
calcareous rocks contain a large amount of chemically
formed carbonate of lime, either of contemporaneous or
subsequent formation, and with this small quantities of
silica or iron are often mingled.
Many calcareous rocks are very pure, but mechanical ad¬
mixtures of clay or sand with the calcareous matter are of
course frequent, and give rise to sundry bthological
varieties.
A Limestone may be defined as a calcareous rock
which is sufficiently coherent and consolidated to be called
stone, though it may be soft or hard. Like all other
rocks, limestones exhibit various degrees of consolidation ;
some are compacted by pressure, others by the infiltration
of a crystalline cement, and, in some cases, the whole
rock has assumed a crystalline structure, which completely
obscures its organic origin. The varieties of limestone are
consequently very numerous, and their differences appear
to depend upon the three following circumstances :—
1. The nature of the organic particles, whether derived
from Corals, Mollusca, Crustacea, Echinoderms,
or Foraminifera.
CHAP. IT.] DERIVATIVE OR STRATIFIED ROCKS. 363
2. The proportion of other mineral matter mingled
with these.
8. The extent of subsequent consolidation by pressure,
infiltration, and crystalbzation.
The following are some of the more important and com¬
mon varieties of limestone.
Shell limestone is a rock in which the shell-fragments
are large and conspicuous, so that the nature of the organic
remains is easily ascertained. Occasionally some particular
fossil is so abundant as to give its name to the rock ; thus
we have the Pentamerus limestone of Shropshire, the
Orthoceras limestone of Sweden, the Hippurite limestone
of Southern Europe, and the Nummulite limestone from
the large Poraminifera called Nummulites.
Crinoidal limestone is composed of broken fragments of
the Echinoderms called Crinoids or Encrinites, and Coral
limestone is similarly composed of the broken fragments of
corals and nullipores (see p. 284).
Chalk is a white, earthy, fine-grained hmestone, some¬
times soft and friable, sometimes hard and compact.
Chemical analysis shows that it is very pure, the whiter
varieties containing from 96 to 98 per cent, of carbonate
of lime. Microscopical analysis shows that it consists
partly of the minute shells of Poraminifera, partly of the
broken fragments of the shells of bivalve Mollusca, the
fibrous sheUs of a particular genus called Inoceramus fur¬
nishing the greater number of these fragments. In some
beds of English chalk these shell-fragments are quite
equal in bulk to that of the Poraminiferal remains, and in
weight would far exceed the latter. Tertiary chalk from
Barbados and the Pacific (New Britain) has a similar com¬
position, but the shell-fragments are fewer and are derived
from thin shells, not Inocerami.
Oolite, or Roe-stone, consists of a number of small round
particles, about the size of the eggs in the roe of a cod-fish.
These httle grains consist of carbonate of lime arranged in
successive concentric coats round some minute particle of
foreign matter which forms a nucleus. This nucleus may
be a particle of sand, or a minute fragment of coral or
any such substance. Some oolites simply consist of these
364
STRUCTURAL GEOLOGY.
[PAET II.
spheroidal grains, loosely agglutinated to one another ; in
others the interstices are filled up by fine-grained, cal¬
careous mud; others are cemented by an infiltration of
crystalline calcite. They usually form freestones, or rocks
■which can be cut with equal facility in any direction;
Bath-stone is a familiar example. The occurrence of oolitic
rock in recent coral-reefs has been mentioned on p. 285.
Pisolite is a variety in which the grains are as large as
peas.
Lithographic limestone is a very pure, fine-grained lime¬
stone, containing as much carbonate of lime as the purer
varieties of chalk, and equally free from grit and other
impurities, but very much harder than chalk.
Marble.—This name is popularly applied to any compact
limestone which is sufficiently hard to take a polish, but
geologists confine the term to limestones which are in a
highly crystalHne condition, owing to contact with igneous
rock, or other causes as yet little understood.
Siliceous limestones contain, as their name implies, a
certain amount of silica difihised through the calcareous
mud. They are always fine-grained and hard, breaking
■with a splintery or conchoidal fracture. Limestones which
contain a small amount of silica and alumina afford the
materials of hydraulic cement, and are called Hydraulic
limestones. According to Gmelin, hydraulic cement is a
pasty admixture of lime, silica, and water, which when
immersed in water is gradually converted into a hydrated
silicate of lime. When the sihca is in the larger propor¬
tion, the stone is called a Cherty limestone.
Argillaceous limestone contains a variable admixture of
clayey matter. Some varieties of hydraulic limestone would
come under this denomination.
Marl is a term which is used very loosely ; some deposits
being called marl which are really clay (see p. 364), and
others being generally called clays which are really marls.
Chalk-marl is a very calcareous marl, or impure chalk.
Clunch is a hard and tough variety of marl. A marly
limestone is often called a Marlstone.
The following are analyses which show the composition
of some of the rocks above mentioned :—•
CHAP. IV.J DERIVATIVE OR STRATIFIED ROCKS. 365
Analyses of Compact Limestones.
Lithographic
Limestone.
Bath
Oolite.
Devonian
Marble.
Wenloc'k
Limestone.
Carbonate of Lime
96-24
94-59
94-00
74-64
Carbonate of Mag¬
nesia ....
•21
2-50
-08
2-51
Silica
3-30
20-03
Oxides of Iron and
Y 2-02
Alumina . . .
j
1-20
1-90
2-38
Water and loss. .
1-53
1-71
-72
-09
100 00
100-00
100-00
99-65
Analyses of Chalky Limestones.
Upper
Middle
Grey
Chalk
Chalk,
Chalk,
Chalk,
Marl,
Kent.
Westbury.
Farnham.
Wiltshire.
Carbonate of Lime
98-52
93-62
85-95
70-80
Carbonate of Mag¬
nesia ....
•29
-18
1-18
1-72
Sulphate of Lime .
-14
-13
•10
2-65
Silica
-65
1-11
9-37
22-80
Oxides of Iron and
Alumina . . .
-40
2-67
1-94
1-02
Alkalies and loss .
—
2-26
1-47
1-00
100-00
99-97
100-01
99-99
Magnesian limestone, or Dolomite, is of variable chemical
composition; the ingredients are the carbonates of lime
and magnesia, but the proportions of these minerals vary
very much. A dolomitic or magnesian limestone generally
has a gritty texture, and is often cellular or cavernous ; its
colour is usually some shade of yellow or brown. When
regularly bedded it often makes a good building stone, but
occasionally it has a tendency to assume nodular and con-
366
STRUCTURAL GEOLOGY.
[PAET II.
cretionary forms. These sometimes resemble cannon-balls
scattered through the rock, and are sometimes grouped
together like bunches of grapes. This nodular structure
may be seen in the cliffs of magnesian limestone near
Sunderland, whence balls from three to six inches in dia¬
meter may be extracted; these consist mainly of calcic
carbonate in crystals which radiate from the centre, while
the horizontal lines of lamination may be traced across
them, showing that the nodules are of subsequent forma¬
tion.
We have seen (p. 287) that some dolomites may have
been formed by direct chemical precipitation, but others
have undoubtedly been produced by the subsequent dolo-
mitization of ordinary limestone. It is probable that this
dolomitization is effected by the percolation of water con¬
taining some salt of magnesia in solution. Cases occur
where a portion only of a limestone mass has been thus
altered, the dolomite occurring as a broad rib bounded by
joint planes ; in other cases a single bed, or group of beds,
has been dolomitized ; the mode of alteration being doubt- •
less determined by the comparative facility with which
water could percolate either down the joints or along the
bedding planes of the rock.
It has been thought that the peculiar structure of mag¬
nesian limestone is due to the diminution in bulk conse¬
quent on the conversion of part of the carbonate of lime
into dolomite. This substance is denser than calcite, having
a specific gravity of about 2"9 compared with 2*7 of calcite,
and its formation may produce a general contraction of
the rock-mass, which results in the development of the
cellular structure.
Tufaceous limestone.—This is always of fresh-water or
terrestrial origin, and has the same variable structure as
recent tufa or travertine (see p. 204), being sometimes friable
and cellular, sometimes hard and rough, and sometimes
partly crystalline.
Bituminous limestone is a black rock containing much
carbonaceous matter derived from the decay of animal
or vegetable matter. Many dark limestones emit a fetid
smell when struck by a hammer, probably from the pre¬
sence of animal matter, and the smell is sometimes so
CHAP. IV.J DERIVATIVE OR STRATIFIED ROCKS. 367
strong tliat the quarries can be smelt at a distance of a
hundred yards. To such fetid hmestones the Germans
give the name of stinkstein.
Glauconite marl or limestone is one containing numerous
grains of the mineral called Glauconite. The basement
beds of the chalk in the southern and central parts of
England are glauconite marls. Bargate Stone and Kentish
Mag are glauconite limestones.
Gypsum, or hydrous sulphate of lime, occurs in layers
and beds of various thicknesses. The thin beds and veins
are generally fibrous, being made up of long acicular crys¬
tals, which are disposed at right angles to the boimding
surfaces ; but in thicker beds the texture is often granular.
The gypsum of Montmartre, from which plaster of Paris is
derived, is chiefly granular, each bed being composed of
many layers of little crystals, slightly differing in colour
and texture, giving the mass a laminated appearance. In
an instance observed by Mr. Jukes at Montmartre, a bed
composed of such crystalline layers was traversed by the
cleavage planes of a subsequent and more massive crys-
talhzation. Anhydrite, the non-hydrated form of calcium-
sulphate, is sometimes foimd, but when exposed it absorbs
water and becomes gypsum.
Although not a calcareous rock, it is convenient to con¬
sider rock-salt here, as it does not come into any of the
other classes.
Rock Salt, or chloride of sodium, is generally found in
lenticular bed-like masses of greater or less extent and
thickness. In England and Ireland it commonly occurs as
a rudely crystalline mass, which is frequently stained red
by the admixture of ferruginous clay ; the beds are often
from 60 to 80 feet thick, but probably thin out in all direc¬
tions. In the Carpathian mountains there are a series of
salt-beds, which in some places make up a thickness of
1,200 feet, and extend for a distance of 500 miles.
4. Ferruginous Mocks.
Ironstone is a popular name for any rock which contains
sufficient iron to be worth smelting, consequently there are
as many different kinds of ironstone as there are of lime-
368
STRUCTURAL GEOLOGY. [PAET II.
stone, and the iron is sometimes in the state of carbonate
and sometimes of oxide.
Argillaceous ironstone, or clay ironstone, consists mainly
of carbonate of iron (60 to 65 per cent.), with varying^
amounts of carbonate of lime, clay, and silt. The coal-
measure ironstones are of this kind ; they occur in nodular
layers and irregular beds, from a few inches to several feet
in thickness. The large, heavy nodules or concretions which
occur in many clays contain much carbonate of iron. The
Blackband ironstones of Scotland consist of carbonate of
iron mixed with 15 or 20 per cent, of carbonaceous or
coally matter, so that they can be calcined with very little
fuel.
Siliceous ironstone.—When the iron-ore is mixed with
sand we have a siliceous ironstone or a ferruginous sand¬
stone, according to the relative proportions of the two
materials. Such a rock is the Northampton ironstone. This
now consists of hydrated peroxide of iron and siliceous
matter ; but the iron was originally in the form of carbon¬
ate, and has been oxidized by the action of percolating
water charged with carbonic and crenic acids (see p. 90).
Ferruginous limestones.—These are rocks which have
originally consisted of the carbonates of lime and iron in
varying proportions, but the action of percolating water
has always oxidized more or less of the iron-carbonate, so
that the surface portions of the rock are decomposed and
reddish-brown in colour ; but when followed below a cover
of other rocks, or met with in boring or sinking, such a
rock is bluish-grey in colour. The Marlstone rock of the
Lias is a ferruginous limestone of this kind, and is often
sufficiently rich in iron to be worked. In the Cleveland
district of Yorkshire it contains from 60 to 70 per cent, of
iron-carbonate. The Tealby ironstone (Lincolnshire) is a
marlstone which is full of small brown oobtic grains of
limonite, and where the stone is soft and less rich in iron
it is locally called Roach.
Hœmatite sometimes occurs in the form of beds and
irregular masses, and is then entitled to be called a rock ;
they generally occur in limestones, and seem to replace
portions of this rock, the iron having been introduced in
solution. In many cases the limestone may have been
CHAP. IV. j DERIVATIVE OR STRATIFIED ROCKS.
369
first converted into dolomite. Sometimes it forms very
large masses, sucli as that of the mountain called Pilot
Knob in the Eocky Mountains.
Chalyhite and Magnetite also sometimes occur in masses
large enough to be called rocks, and occasionally form the
substance of whole mountains. The marlstone rock above
mentioned is often 20 to 80 feet thick, and the Erzberg at
Eisenerz, in Styria, which rises 2,600 feet from its base,
consists almost entirely of chalybite. The Magnetberg in
the Ural Mountains is wholly composed of magnetite, and
Gellivara in Lappmark, a mountain 2,000 feet high and
three miles long, also consists of magnetite.
5. Carbonaceous Bocks.
Coal may be defined as an accumulation of mineralized
vegetable matter, which has lost much of its oxygen, and
has been compressed into a rocky substance. In chemical
composition it differs from wood chiefly in having a less
proportion of oxygen and a greater proportion of carbon.
In wood, the average proportions of carbon and oxygen are
respectively 49 and 43 per cent, of the whole mass.
Lignite is altered and compressed wood in the first stage
of becoming converted into coal; it is often quite black,
but stiU soft and woody, and contains from 22 to 25 per
cent, of oxygen.
Brovm coal may be regarded as compressed and partially
carbonized peat, but there is often much lignite mixed
with it. In colour it is generally brownish and soft when
quarried, but sometimes it is compact and nearly black.
Its vegetable origin is generally apparent.
Cannel or Parrot coal is compressed vegetable matter
which has lost still more oxygen, retaining only some 10 or
12 per cent. It has a platy fracture, and is generally bright
and shining, but sometimes dull and earthy, its appearance
depending on the amount of dust and ash it holds. It does
not soil the fingers, and always bums brightly, hence the
name of " cannel," or candle-coal. It often takes a good
polish, and can be worked like jet, which is, in fact, a
similar substance found in smaller quantities.
Common house-coal is hard and compact, though often
B B
370
STRUCTURAL GEOLOGY.
[part ii.
brittle, breaking generally into cuboidal lumps. There
are three marked varieties: (1) Caking coal, which runs
together into clinkers or cinders : the Newcastle coals are
of this kind. (2) Splint or Hard coal, common in Scot¬
land, not easily broken or kindled. (3) Cherry coal, which
is lustrous, easily breakable, quickly kindled, and does not
cake. To this variety most of the StafEordshire coals
belong.
Anthracite or Stone-coal is very hard and heavy, with a
glossy lustre, and a more completely mineralized appear¬
ance.
The transition from Lignite to Anthracite by the gradual
removal of the oxygen is shown in the following table :—
Lignite.
Earthy
Cannel
Coal.
Splint
Coal.
Caking
Coal.
Anthra¬
cite.
Carbon ....
Hydrogen . . .
Oxygen....
Nitrogen . . .
Ash
65-3
6-6
1 25-3
21
66-4
7-54
10-84
1-36
13-82
75-58
5-50
8-33
1-13
5-46
84-28
5-52
6-22
2-07
1-89
91*44
3-46
2-58
0-21
2-31
Bitumen or Asphalt is a light, brittle, black, or nearly
black mineral, some varieties much resembling cannel
coal, but differing in being soluble in turpentine and in
melting under the influence of heat. It contains much
more hydrogen than coal, and belongs to the group of
substances known as Hydrocarbons. It occurs in lenticular
lumps and masses among shales and flaggy sandstones,
and sometimes forms layers of considerable extent, as in
the island of Trinidad (West Indies).
Petroleum is a liquid hydrocarbon, some varieties being
thin and clear, others thick and viscid, bke tar. Being
liquid, it is not confined to the rocks in which it was first
generated, but makes its way along cracks and fissures,
and even rises to the surface in the manner of an artesian
spring. Hence it occurs in rocks of all kinds and ages.
Some shales and sandstones hold large quantities of it.
CHAP. IV.J DERIVATIVE OR STRATIFIED ROCKS. 371
and shale which is so saturated as to yield oil on slow
distillation is known as oil-shale. A good oil-shale may be
Tmown by the peculiarity that parings made with a knife
curl up as they are cut. Oil-shale is generally rich in
organic remains, especially of fishes and molluscs, and it
seems probable that in most cases the oil has been derived
from the decay of these creatures. The richer shales yield
from 80 to 40 gallons of mineral oil per ton of shale.
CHAPTER V.
STRATIFICATION.
HE Formation of Strata.—The descriptions given
in the first part of this book of the manner in which
deposits are formed in the rivers, lakes, and seas of the
world are really descriptions of the formation of strata, and
we have throughout assumed that the reader has a general
comprehension of what is meant by lamination and strati¬
fication, so that it is not now necessary to enter into any
elaborate definition of these terms.
It is sufficient, therefore, to say (1) that stratification is
the airangement of deposited matter in regular layers or
strata ; (2) that lamination is a structure possessed by cer¬
tain strata which separate into still thinner layers or
laminae, each of which is a separately deposited film of
sediment.
Laminae and strata are formed in exactly the same way,
by the deposition of successive layers of material, and the
difference between them is simply that of the thickness
accumulated between the pauses of deposition ; these pauses
being marked by the planes along which they separate.
All stratified rocks, whether coarse or fine in texture,
split into beds or strata, which may be of any thickness
from an inch to several feet ; but it is only rocks of fine tex¬
ture which split into laminae. In some cases fifty or sixty
separate laminae may be counted in the thickness of an
inch ; in others the laminae are only discernible as bands of
colour, or as forming a kind of grain in the rock, so that it
splits most readily in a direction parallel to the planes of
bedding.
These general facts of lamination and stratification are
CHAP, v.]
STRATIFICATION.
373
represented in fig. 88, where seven different beds succeed
one another in regular order ; the dotted beds being meant
for sandstones, the lined beds for shales, and the plain beds
at the top for limestones. In the limestones no lamination
is visible ; in the sandstones it is just visible by the
arrangement of the component particles along lines which
produce a definite " grain " ; lastly, the beds of shale are
fissile or divisible into separate laminae.
Since in one bed or stratum of shale there are a certain
number of laminae, say, for instance, it is composed of fifty
such laminae, and since each of these was separately de¬
posited, it follows that so many separate acts of deposition
took place in the making of that single stratum, and that
its production was a process involving a considerable
amount of time.
Lamination does not depend on the fineness of the mate¬
rial, nor altogether on the presence of flaky minerals such
as mica, but chiefly on the manner in which the material
was spread out. The want of lamination may be due to
the continual subsidence of flne sediment for a long time
without any pause in the deposition, so that the whole
forms an inseparable bed or mass of clay or marl ; or it
may be due to the rapid accumulation of a large quantity
of sediment at once, the result, perhaps, of a single flood,
as in the case of some beds of sand or gravel.
Length of the Intervals between Beds.—The in¬
tervals or periods of non-deposition marked by the planes
of separation between beds must vary considerably in
374
STRUCTURAL GEOLOGY.
[PAET II.
length. The interval between the formation of two con¬
tiguous beds of similar lithological character is not likely
to have been so long as that between beds of different
lithological character. In a series of sandstones, for
example, the pauses between the deposition of the succes¬
sive beds may not have been longer than the intervals
between successive tides, and it is clear at any rate that
the same physical conditions which favoured the deposition
of the first beds were continued during the deposition of
the succeeding beds.
On the other hand, in such a series of strata as those
represented in fig. 88, where each superimposed bed is
different to that on which it rests, the intervals between
the beds represent pauses during which some change took
place in the surrounding physical conditions. Thus, in
the case of a shale resting on a sandstone, we must sup¬
pose some alteration to have occurred either in the velo¬
city of the currents, or in the geography of the district.
The change may have resulted from a diminution in the
strength of the current, so that it could only transport mud
to the place where it previously brought sand ; or from the
introduction of a fresh current carrying mud only instead
of sand ; or, lastly, from the depression of the land whence
the supply was obtained, so that the distance from the
shore was increased, and only finer sediment could be
deposited.
In the case of a sandstone succeeding shale, we should
assume the reversal of one of the causes above mentioned,
either an increase in the velocity of the current, or the in¬
gress of a sand-bearing current, or the general elevation of
the coast, which would probably change the set of the cur¬
rents altogether. A limestone resting either on shale or
sandstone would lead us to conclude that similar changes
were carried on to a still greater extent, till they resulted
in the cessation or diversion of all mud-bearing currents,
and a long period of rest or slow subsidence ensued, during
which the limestone was slowly formed by the accumula¬
tion of shells and other calcareous structures secreted by
the various animals which lived in the clear waters.
A shale overlying a limestone is, of course, the reverse
of the case just mentioned, and the invasion of clear water
CHAP, v.]
STRATIFICATION.
375
by a mud-bearing current. Instances sometimes occur
where the results of such a change are very apparent.
Thus at Bradford, near Bath, the section represented in
fig. 89, a, may be seen ; the lower part being a limestone, on
the surface of which there once grew a colony of the peculiar
animals called Encrinites, represented in fig. 89, h. When
alive these animals were fixed to the sea-bottom by a solid
calcareous base, and on the surface of the limestone these
bases are found in great numbers, while scattered in the
clay above are remains of the stems and heads of the
Encrinites, belonging to individuals of all sizes and ages.
Fig. 89, a. Fig. 89, b.
a, Beds of limestone covered by h, Restoration of Encrinites as
clay, containing Encrinites. they lived on the sea bottom.
Now it is plain that in this case, after the limestone was
formed, there was an interval during which the sea was
quite clear and free from sediment, and therefore well
adapted for the growth of these particular kind of crea¬
tures, which cannot live in muddy water. They settled on
the limestone, and flourished there for a sufficient period
of time to allow of successive generations arriving at matu¬
rity undisturbed, until, finally, the sea-bottom was invaded
by a current of muddy water, which killed them, and buried
their remains in a deposit of mud or clay.
Another example in which the succession of events can
be clearly traced is illustrated by fig. 90, which represents
part of a quarry in the Isle of Portland.
376
STRUCTURAL GEOLOGY.
[part ii.
The lowermost beds in this quarry consist of the Port¬
land Oolite (1, 2), full of marine shells ; upon this marine
limestone rests a thin seam of black earth (3), full of vege¬
table remains. Above this are beds of limestone (4), con¬
taining fresh-water fossils, and this is succeeded by a
thicker seam of vegetable earth (5), full of the roots,
trunks, and branches of ancient trees ; this is locally known
\ 8. Shaly limestone.
y 7. Clay parting.
6. Shaly limestone.
Fig. 90.
Purbeck Beds at Portland.
30 feet.
' Dirt bed " with
tree stumps.
4. The " Cap " lime¬
stone.
3. Dirt bed.
2. Skull cap.
Thin layer of clay.
1. The Roach (Port¬
land).
Total thickness about
as the " dirt-bed." Above come finely laminated lime¬
stones (6, 7, 8), of fresh-water origin.
The most remarkable fact here exhibited is the position
of the trees in the dirt-bed, for they are still erect in the
position of growth, with their roots in the vegetable soil
and their trunks extending into the limestone above. The
interpretation of these facts is as follows :—
The sea-bottom, represented by beds 1 and 2, was ele¬
vated into dry land on which many kinds of plants grew
CHAP. V.J
STRATIFICATION.
377
in great luxuriance and contributed their remains to the
formation of the vegetable mould (3). A change then
occurred which converted this part of the terrestrial sur¬
face into a fresh-water lake, and led to the deposition of
the limestone (No. 4). This lake, however, gradually silted
up, and part of it passed into the condition of a swamp or
fen, which was favourable to the growth of an extensive
forest. The lapse of time represented by the accumula¬
tion of this little bed of 12 inches in thickness must be
very great : time for many generations of trees to grow,
flourish, and decay.
Dr. Mantell has thus described the scene presented by
the quarry in 1832, when a large surface of the dirt-bed
had been cleared for removal.' " The floor of the quarry
was literally strewn with fossil wood, and before me was a
petrified forest, the trees and the plants, like the inhabi¬
tants of the city in Arabian story, being converted into
stone, yet still remaining in the places which they occupied
when alive The upright trunks were generally a few
feet apart, but 3 or 4 feet high ; their summits were broken
off and splintered, as if they had been snapped or wrenched
off by a hurricane at a short distance from the ground.
Some were 2 feet in diameter, and the united fragments of
one of the prostrate trunks indicated a total length of from
30 to 40 feet. In many specimens, portions of branches
remained attached to the stem. The external surface of all
the trees 1 examined was weather-worn, and resembled that
of posts and timbers of groins or piers within reach of the
tides, and subjected to the alternate influence of the water
and atmosphere ; there were but seldom any vestiges of
the bark."
It is plain that the forest was destroyed by the depres¬
sion of the district, and the consequent influx of the lacus¬
trine waters. Layer after layer of calcareous mud was de¬
posited over stems and roots, until the old forest was
buried under many feet of limestone. To accomplish this
change must have taken a considerable length of time.
The Law of Vertical Succession or Superposi¬
tion.—The simple facts which have just been described
' " Wonders of Geology," seventh edition, p. 400.
378
STRUCTURAL GEOLOGY.
[part ii.
form the basis upon which the whole fabric of historical
geology is founded. In any succession of beds each one
represents the conditions which prevailed over a certain
area for a certain length of time ; the lowest is the oldest,
the uppermost is the newest, and the relative age of the
others is indicated by their relative position. In order to
mark this fact, geologists are in the habit of numbering
such a succession of strata from below upwards, so that if
the beds represented in fig. 88 were observed in a chff they
would be described as follows :—
No. Beds. Thickness.
7. Compact limestone ... 3 feet.
6. Argillaceous limestone . . I „
5. Laminated shale .... 4 „
4. Flaggy sandstone .... 3 „
3. Laminated shale .... 3 „
2. Fine-grained sandstone . . 2 „
I. Sandy conglomerate . . . 3 „
Alternation and Inter-stratification of Beds.—A
series of beds which have been uniformly and consecutively
deposited generally preserve a certain regular order in their
vertical succession. Beds of very fine and very coarse
texture are seldom in contact with one another, except
where the lower stratum has suffered erosion before the
deposition of the upper stratum, which then rests upon a
surface of denudation. In a natural and normal series,
therefore, deposited during a period of continued depres¬
sion, conglomerates are covered by sandstones, sandstones
are succeeded by shales or clays, and these are followed by
marls or limestones, as in fig. 88.
Similarly, if a marine area, on the bottom of which
calcareous deposits have been formed, is affected by a move¬
ment of elevation, the clear water is first invaded by gentle
currents which introduce finely divided mud before it is
traversed by those of sufficient strength to carry the coarser
material of sand. Pauses in this process, or alternate move¬
ments of elevation and subsidence, will of course occasion
corresponding alternations in the succession of deposits.
It is seldom indeed that the change from one kind of
deposit to another is sudden and complete ; where a sand-
CHAP. T.J
STRATIFICATIOX.
379
stone passes vertically into a shale, there is usually a space
where beds of shale and sandstone alternate with each
other, or where laminated sandstones form a passage from
the arenaceous to the argillaceous strata. Similarly, where
a shale is succeeded by a limestone, the lower beds of the
latter are usually separated by layers of shale. Two kinds
of rock occurring alternately in this way are said to be
interbedded or interstratified with one another, as in the
case of the limestones and dirt-beds in fig. 90.
In such an alternating series we generally find that there
is a certain association between beds of finely divided and
slowly accumulated matter, and a similar association of the
coarser and more rapidly formed sandstones, pebble-beds,
and conglomerates.
This association of beds is particularly well exemplified
in the coal-measures, where a bed of coal almost invariably
rests upon a layer of fine clay representing the argillaceous
soil upon which the coal-plants grew. This layer is often
termed the underclay in England, and the coal-seat in the
south of Ireland. The general order of superposition is,
(1) Sandstone ; (2) Underclay ; (3) Coal ; (4) Shale. The
roof, or argillaceous bed above the coal, is always more
shaly than the underclay, and there is often shale below
the latter separating it from the sandstone. This order of
succession is illustrated by the following example taken
from one of the memoirs of the Geological Survey : '—
No.
Beds.
Feet.
15.
Argillaceous shales
54
14.
Coal and shale
4
13.
Coal .
1
12.
Underclay
4
11.
Argillaceous shale.
4
10.
Sandstone
2
9.
Argillaceous shales
23
8.
Coal
9
7.
Underclay
3i
6.
Coal
OJ
5.
Underclay
2
4.
Argillaceous shales
7
3.
Sandstone
1
2.
Argillaceous shale
2
1.
Sandstone
6
^ " Mem. Geol. Survey," vol.
i. p. 210 (Bristol coalfield).
380
STRUCTURAL GEOLOGY.
[PAET II.
Whether any particular bed is thick or thin depends
upon the continuance or interruption of the conditions
which led to its formation. For the formation of a great
thickness of one kind of rock, it is probably necessary
either that the sea in which it is being formed should be
very deep, or that the area of deposition should be slowly
and continuously subsiding, without causing any material
alteration of the currents emanating from the sources of
supply.
Thick accumulations of one particular kind of matter
are by no means rare. Sandstones often occur in an un¬
interrupted succession of beds for many hundreds of feet,
exhibiting every variety of texture, from coarse pebbly beds
to layers of the finest possible grain, but without any
appreciable admixture of argillaceous matter. Similarly,
beds of clay or shale often form a continuous series of
great thickness with hardly any intercalations of sandy or
calcareous layers. The thickness of certain limestone for¬
mations is even greater, amounting, in some cases, to
thousands of feet. Thus the chalk of England is in some
places more than 1,000 feet thick, and the dolomite of
South Tyrol has a thickness of from 3,000 to 5,000 feet.
Horizontal Extent and Thinning out of Beds.—
The manner in which mechanical deposits are formed was
described in Part I., Chap. XIV., and it was especially
pointed out that such deposits generally preserved a re¬
gular order of succession from the coast outwards, the
size of the transported particles decreasing with the dis¬
tance from land, the finest muds being last deposited, till
a limit is reached beyond which no such transported
material is found, and its place is taken by sediment of
organic origin.
It is clear, therefore, that where such a series of deposits
as that represented in fig. 64 has been consolidated and
elevated into land, a geologist engaged in tracing the beds
across country in one direction, would find them rapidly
changing in lithological character ; the conglomerates will
consist of smaller and smaller pebbles till they pass into
coarse sandstones, the sandstones will become gradually of
finer texture till, if he can trace them far enough, they
will pass into beds of shale or clay. This change some-
CHAP. V.J
STRATIFICATION.
381
times takes place rapidly, and within the compass of a
single group of beds; in other cases, beds of sandstone
alternate with the conglomerates, and as the latter gradu¬
ally diminish in thickness, the former thicken and take
their place. Similarly beds of shale may alternate and
come in between beds of sandstone, and where the shales
begin to thin out, beds of limestone may commence and
finally take their place.
In one locality, therefore, we may find a series of lime¬
stones which may extend over a large area, but when
traced in one direction will begin to be separated by little
partings of shale. On following the group of limestones
and shales still farther we might find that the shales be¬
came thicker and the limestones thinner as we proceeded.
Finally some of the limestones might thin out and termi¬
nate altogether, while beds of sandstone commenced to
appear between the shales, until at length the series might
consist almost entirely of shales and sandstones with only
one or two beds of limestone to represent the purely cal¬
careous group with which we commenced. Under these
circumstances we should conclude that we had been ap¬
proaching an ancient shore-line. Fig. 91 may serve as
a representation of the way in which such lateral changes
take place, the white bands being meant for limestones,
the black for shales, and the dotted bands for sandstones :
the figure, however, is a mere diagram, and would have
to be drawn out to twenty or thirty times its length
before it could be taken as a representation of natural
deposits.
Every bed is in fact a great lenticular cake which thins
out and terminates somewhere in every direction. As a
general rule there is a definite relation between the extent
and composition of beds, viz., that the finer the material
Fig. 91. Diagram of Lateral Change.
382
STRUCTURAL GEOLOGY.
[PAET II.
of which a bed is composed, the wider is its extent and
the more equable its thickness. Individual beds of con¬
glomerate and sandstone are generally thicker than single
beds of shale or limestone, but they thin out much more
rapidly, while beds of limestone and coal are often per¬
sistent over large areas, though they may be only a few
feet in thickness.
Mr. Jukes observes that *' the extent of single beds is
most certainly ascertained in coal mining, in which the
horizontal or lateral extension of beds is followed. Par¬
ticular beds of coal, or of shale, or other rock having
recognizable characters, are sometimes known to spread
throughout a whole district. For instance, in South
Staffordshire, a bed of smooth black shale, a little below
the Thick or Ten-yard coal, is known as the ' Table batt.'
It has a thickness of from two to four feet, and extends
over all the greater portion of the South Staffordshire
coal-field—places where it is known being ten or twelve
miles apart from each other in different directions. Its
original extension was probably much greater, since the
beds now disappear in one direction by ' cropping out,' and
are buried in others at too great a depth to be followed.
Known beds of coal, with a particular designation, such as
' Heathen coal,' extend over still wider areas, and similar
facts occur abimdantly in most coal-fields. Mr. Hull
states that one bed of coal called in part of the Lancashire
coal-field the ' Arley Mine,' but known by other names in
other parts, spreads over the greater part of the coal-field,
which has an area of 192 square miles." '
On the other hand, beds of sandstone in the same coal
districts frequently thicken or thin out very rapidly. An
instance occurs near Wednesbury, in South Staffordshire,
where a bed of sandstone known by the name of the
New Mine Kock " thickens out from 9 feet to 78 feet in
the course of a few yards horizontal distance. In other
parts of the district the sandstone varies from 60 to 15
feet, and in some places is entirely wanting.
A remarkable example of the persistence of the coal-
seams for great distances and the impersistence of the
' " Manual of Geology," second edition, p. 185.
CHAP. T.J
STRATIFICATION.
383
mechanical deposits is found in another part of the same
coal-field. At Essington there is a certain group of " Coal-
Measures," consisting of alternations of sandstones, shales,
and coals, and attaining a thickness of between 300 and
400 feet. This group, when traced towards the south,
thins so rapidly by the gradual dying away of the shales
and sandstones, that in the space of five or six miles the
different beds of coal come to rest directly one upon the
other, and are continued as a compound seam of coal, 30
feet thick, with only a few shaly partings between the
beds.^
Groups and Episodes.—From the explanations and
examples given in the preceding pages it will be evident
Fig. 92.
a. Thick coal. h, Thick coal group at Essington.
c. Heathen coal.
that when geologists speak of a group of beds, they do not
necessarily mean a set of beds of similar lithological com¬
position, Wt a set of associated beds, connected together
both vertically and horizontally, and deposited in con¬
tiguous areas during any given period of time. Such a
series is found in the Carboniferous or Mountain limestone
group. In Derbyshire this consists almost entirely of
limestones, which succeed one another in massive beds for
a thickness of more than 4,000 feet. When traced north¬
wards, however, into Yorkshire, this great group is
gradually replaced by a series of shales, limestones, and
sandstones, which form several minor groups or divisions
of the series.
^ " Mem. Geol. Survey, S. Staffordshire Coalfield," second edition,
p. 19.
384
STRUCTURAL GEOLOGY.
[part ii,
Jukes long ago remarked' that "it occasionally happens
that one large series of beds having a common character
includes in some part of it a small, distinct set of beds
with a peculiar character of their own," and he observed
that some definite term was wanted to designate such an
included set of beds. The name he then proposed has,
however, been appropriated to express a different idea, and
Prof. J. P. Blake has recently introduced the term episode,''
which may be adopted as a convenient word for indicating
the relation above mentioned.
An episode then may be defined as a small group or set
of beds of limited extent possessing special characters
which differ from those of the normal series within which
it is included. It is, in fact, the lenticular development of
a special set of beds formed under exceptional conditions,
and therefore differing from those which were formed
elsewhere at the same time.
Current Bedding. (1.) Diagonal or Obligue Bedding.—
In fig. 82 the beds are represented as horizontal, each
stratum resting evenlj'' on that below ; but stratification is
not always so regular as this, because the surfaces on which
beds are laid down are not always so even and horizontal.
For instance, if a current is running over a surface which
ends in a slope, the sand, which is being drifted along the
bottom, will roll down the slope and remain undisturbed.
Layer after layer of sand may thus be deposited in an
inclined position according to the slope of the bank.
Such a set of inclined strata is often formed where a river
enters a lake, as described on p. 235, and is perhaps more
frequent in delta deposits than any others, because river
currents are, as a rule, more steady and continuous in one
direction. An instructive instance has been described by
Mr. Jukes in his memoir on the South Staffordshire coal¬
field. In this case observations in several quarries over an
area of at least a quarter of a square mile showed that the
beds of sandstone were inclined at an angle of 30°, and it
was thought at first that this inclination was due to a sub¬
sequent tilting of the strata until another cutting was
found, which showed that they rested on a horizontal bed
^ " Manual of Geology,'' second edition, 1862, p. 201.
* "Brit. Assoc. Rep. (Sheffield)," 1879, p. 33.
CHAP. T.]
STRATIFICATION.
385
of coal, and tliat it was merely a case of diagonal bedding.
Similarly, near Corsham, in Wiltshire, a group of Oolitic
limestones may be seen resting obliquely on other beds of
shelly limestone, which are nearly horizontal.
(2.) Curvilinear or Irregular Bedding.—This structure
is produced under similar circumstances to that just
described, but should be distinguished from it, inasmuch as
it is always a proof of frequent change in the velocity and
direction of the currents by whose action the deposits were
, accumulated. Diagonal bedding is produced by the action
of a steady current flowing continuously in one direction.
Curvilinear bedding is caused by the meeting of currents
which vary in force and direction from time to time. The
term False-bedding has been applied to both structures,
but is a misleading term, for both are truly bedded,
though the inclination of the beds may be deceptive.
Sir Charles Lyell has described the appearance of a cut-
ting through a sandbank formed near a junction of the
rivers Arve and Ehone, where the conflicting currents had
produced a series of inclined beds with curvilinear sur¬
face ; ^ above these were more or less horizontal layers,
surmounted by irregular alternations of sand and gravel
in undulating curvilinear layers. Irregular bedding of
this nature is the result of constantly shifting and con¬
flicting currents, and is seldom exhibited by other deposits
than those formed in shallow water, viz., sand and gravel,
or sandstone and conglomerate, though it is not unfre-
quently developed in certain limestones which chiefly
consist of the rolled and drifted débris of shells and corals,
as, for instance, the great Oolite limestone of Bath and
Minchinhampton, and the rock known as the Lincolnshire
limestone.
rig. 98 represents the arrangement of the beds in the
face of a quarry at Minchinhampton, as sketched by Mr.
Jukes.'' Part only of the quarry face is shown in fig. 93,
the top line being a marked plane of bedding ; each bed is
obliquely laminated, but the surfaces of the beds are all
more or less curvilinear ; it is evident also that none of
these beds are complete, but are portions of so many banks
' See fi^re in Lyell's " Principles of Geology," ch. xix. fig. 44.
' See "Popular Physical Geology," Jukes, pi. 8.
c c
386
STRUCTURAL GEOLOGY. [PART II.
of coral sand formed by the agency of local currents which
frequently changed their direction, and constantly removed
and rearranged the materials they acted upon. The con¬
trast and variety is often heightened by differences in the
size or colour of the particles composing the different beds.
Current Mark or Ripple Drift.—Another effect of
current-action is to be found in ripple-marked surfaces.
The rippled surface is similar to that so often seen on the
sands of a sea-shore, where it is produced, not by the
rippling of the waves as they break on the strand, but by
the action of the tidal current which has passed over it,
Fig. 93. Curvilinear Bedding in Great Oolite, Minchinhampton.
and because any current passing over a yielding surface
develops an undulating movement. Wind moving over
the surface of water causes a ripple on that surface.
Water moving over the surface of fine sand causes a similar
ripple upon it.
The formation of ripples on the sandy bottom of a brook
has been described on p. 130, and any current running
steadily in one direction produces similar ripples, the par¬
ticles on the top of each ridge being continually pushed
over its edge, while others are pushed up the slope behind.
In this way a succession of little inclined laminae or
stratulœ are formed, arranged in a series of parallel bands
or layers which slope slightly towards the direction from
which the material is brought, and consequently in an
CHAP. V.J
STRATIFICATION.
387
opposite direction to the inclination of the small stratulse
of which they are composed (see fig. 94). Each of these
bands ends in a surface ridge or ripple.
Mr. Sorby states that in fine-grained sandstones which
exhibit ripple-drift, the small stratulse of sand are often
separated by thin layers of argillaceous matter, the diffe¬
rent bands being also separated by a similar deposit. He
also finds a similar structure in beds of mica-schist, where
the quartzose folia correspond to the sandy layers and the
micaceous to the argillaceous layers, and showing that
these schists were originally beds of shaley or flaggy sand¬
stone.
An oscillating movement will also produce a ripple-
marked surface, though not a regular succession of ripple-
drift layers. This can be tested by oscillating a basin of
water into which sand has been thrown. Waves produced
by the wind produce an oscillating disturbance of the
m m
Fig. 94.
m. Ripple marks, h, Ripple-drift layers or laminae.
material on the sea-floor, and as the influence of large
waves extends to a considerable depth, ripple-marks may
be produced in any depth at which this influence is appre¬
ciable. Tidal currents also disturb the bottom in fairly
deep water under certain conditions (see ante, p. 182).
Eipple-mark has in fact been observed beneath 50 to 60
feet of water.
Mr. Jukes observes that in places where the current is
troubled, "a modification of these rippled surfaces is some¬
times produced, the bed being irregularly mamillated on
its surface, which is pretty equally, though irregularly,
divided into small hollows and protuberances of a few
inches diameter. This surface structure may be seen in
process of production now on shores where spaces of sand
are enclosed by rocks, so that as the tide falls it is made to
run in different directions among the rock channels ; but
it would probably be caused at any depth at which a
388
STRUCTURAL GEOLOGY.
[part ii.
current could be similarly troubled and confused. This
might be called ' Dimpled current-mark.' "
It must be borne in mind that the rippled surfaces which
are formed between tide-marks can seldom be preserved,
because the incoming tide generally obliterates those pre¬
viously formed. It is probable, therefore, that the ripple-
marks preserved on ancient rock-surfaces have in most
cases been formed beneath shallow water at a greater or
less distance from low-tide level. A ripple-mark produced
by the tidal current may be covered with the sediment
which is deposited during the slack of the tide, and the
new surface thus formed may be similarly rippled by the
gentle movement of the return current, so that every suc¬
cessive layer of lamina will be ripple-marked. In fine¬
grained sandstones or sandy shales it is not unusual to find
a succession of ripple-marked surfaces, one imder the other,
at distances of a few inches apart ; and the direction of
the ripples somewhat varies considerably on the different
surfaces.
Mr. Sorby has shown that inferences may be drawn
from the examination of these current-marks as to the
velocity and direction of the currents which caused them,
and that we may thus reason back to some conclusions
regarding the physical geography of the district at the
time they were formed.
Although, as above stated, ripple-marked beds are not
usually such as were formed between tide-levels, yet sur¬
faces are occasionally found which exhibit not only ripple-
marks, but also sun-cracks, rain-prints, worm-tracks, and
the footmarks of the various creatures which travelled
over the moist, exposed surface of the bed. Such surfaces
have doubtless been formed on flat shores, where large
tracts were exposed between the lines reached by neap and
spring tides, so that the surface beyond high water of
neap tides, being exposed to the action of the sun, became
cracked and hardened, and the next spring tide only spread
over it another layer of sand or mud. Among lacustrine
deposits they have doubtless been formed during dry sea¬
sons, when the lake waters shrank and large surfaces of the
lake-floor were temporarily exposed.
CHAPTER VI.
CONSOLIDATION OF BOOKS.
IN the first part of this volume (Chapters XI. to XVI.)
the manner in which deposits are now being formed in
rivers, lakes, and seas has been explained ; these being for
the most part loose and unconsolidated sands, gravels, clays,
and muds. In Part II., Chapter IV., the principal sedi¬
mentary rocks which are found in the earth's crust have
been described, many of these being very hard and com¬
pact materials. It is natural that at this stage the student
should ask how it is that deposits which were originally
soft and incoherent, like most of those now being formed,
have become so hard and consolidated as to be termed in
ordinary language rock or stone. In the present chapter
we shall endeavour to give some answer to this question.
The processes by which stratified rocks are consolidated
may be mentioned under the following heads :—
1. Desiccation.
2. Pressure.
3. Infiltration.
4. Chemical change.
6. Heat.
1. Desiccation.—All stratified rocks which.have been
formed at the bottom of lakes and seas must of course be
saturated with water for a long time after their deposition,
as long, in fact, as the lake or sea remains above them.
When at length earth-movements take place, and the sub¬
aqueous deposits are converted into land, they are partially
drained of the included water, and their materials conse¬
quently settle down into a smaller space.
390
STRUCTURAL GEOLOGY.
[part II.
To take a simple instance, let us imagine a large and
fairly deep lake whicli has during a long period received
the sediment carried in by rivers and brooks, and in which
several hundred feet of stratified deposits have been accu¬
mulated. Let us suppose that the country is then slowly
elevated, and that the channel of the excurrent river is
gradually deepened, so that the lake is eventually drained
and its floor exposed. The main river which once flowed
into the lake will then cut a course for itself through the
lacustrine deposits, and the upper portion of these will he
gradually dried or desiccated, the water in them being
drawn out partly by evaporation from the surface, and
partly by actual flow toward the river-channel, which will
act as a natural drain.
The abstraction of water will cause a contraction of each
layer or bed, as may be seen in the mud of a dried-up
pond. Near the surface this contraction is mainly in
lateral directions, generating
cracks which are rifts at the
surface, hut get narrower and
narrower as they extend down¬
ward into the mass ; the reason
of this being doubtless that the
pj gg layers below contract less hori¬
zontally than they do vertically,
owing to the pressure exercised by the weight of the layers
above them. Partly then by contraction, and partly by
the weight with which each successive stratum bears on
that below it, the particles of every stratum are compressed
into a smaller space, and are forced to cohere more closely
together. This is generally the first stage in the process
of consolidation.
An excellent illustration of the amount of contraction
and compression which a calcareous mud often undergoes
in the process of conversion into a limestone is afforded by
a bed in the Lower Purbeck group of the Vale of Wardour.
This is a pale yellowish-grey earthy limestone, containing
many small sheUs of species that can live in brackish
water, and exhibiting many curious cavities of the shapes
shown in fig. 95, some of them empty, and some of them
filled with ferruginous matter. These are spaces left by
CHAP. VI. J
CONSOLIDATION OF ROCKS.
391
the solution of cubical crystals of common salt ; they show
that when the deposits were raised into dry land, the salt
was dissolved and removed, and that the material con¬
tracted by desiccation under the pressure of the overlying
strata (some 200 feet) to such an extent, that the walls of
the canities were forced inwards. The squeezed-up shapes
of the cavities show that this must have taken place while
the mass was soft and plastic, before it set into the hard
limestone it now is. In some of them the walls have " caved
in " equally, but in many two sides have given more than
the rest, while sometimes they are all so bent in that little
space remains between them.
2. Pressure.—In the case taken above we have supposed
the thickness of the lacustrine deposits to be only a few
hundred feet, but the geologist often finds marine strata
which form a continuous series for several thousand feet,
and it is obvious that the lower parts of such masses
must have been subjected to considerable pressure from
the mere weight of the overlying beds. Further, where
such thick formations rest on still older rocks, these latter
must have borne the whole weight of the newer series.
For example, the thickness of the Carboniferous system
in Lancashire is estimated at about 20,000 feet, and the
whole of this great pile of sediment was accumulated
in horizontal layers in a subsiding area. When complete
this great formation must have borne with its whole weight
upon the underlying rocks, and the vertical pressure thus
exercised must have been very great.
Again, in subsequent chapters proofs will be given that,
during the movements which have taken place in the crust
of the earth, stresses have been set up which have subjected
certain regions to enormous lateral pressures. It will be
shown too, that such compression has not only consolidated
the rocks by forcing their component particles into closer
collocation, but has actually caused these particles to move
and rearrange themselves in accordance with the direction
of the stress, thus developing new and peculiar structures
in the rocks exposed to its influence.
The results of simple pressure are illustrated by several
processes in arts and manufactures ; thus, the black lead
of pencils is formed by compressing the powdered dust of
392
STRUCTURAL GEOLOGY.
[part II.
graphite, and tiles or tesserae have been made by the com¬
pression of porcelain-earth.
More astonishing, however, is the power of suddenly-
applied pressure to consolidate instantaneously such a
material as loose sand. This is demonstrated by arrange¬
ments made for trying the strength of gunpowder. A
cannon-ball is fired from a gun charged with the powder
to be tested at a mortar in which leathern bags filled with
sand have been placed to receive it. The sand is often
compressed by the percussion of the ball into a mass of
sandstone sufficiently firm to remain solid and to bear
handling.
Many sandstones have been compacted by simple pressure,
their component particles having been so closely packed
by this process that they adhere to and support one
another ; a slight scattering of mica flakes among the sand
greatly facilitates its acquisition of coherency. Soft sand¬
stone which just holds together in this way is sometimes
called " sand-rock," and if it is firm enough to be cut into
blocks these sometimes show a certain amount of flexibility ;
for if a long block of the stone is supported at both ends
it will curve and sag down in the centre. Such is the
Itacolumite or Flexible Sandstone of India and Brazil.
There is no more striking proof of the enormous pressure
to which some rocks have been subjected than that fur¬
nished by certain conglomerates, the pebbles in which have
indented each other. In these beds, pebbles of the hardest
rocks, such as Quartzite, are dented and squeezed by
compressive contact with one another, as if they had been
lumps of dough or putty manipulated by human fingers.
Some pebbles may be found among them which have been
split into several pieces, but held together and subsequently
recemented by infiltrated silica or calcite.
3. Infiltration.—In many cases the consolidation of a
rock is assisted or completed by the infiltration of some
mineral cement. It has been stated that rain-water per¬
colating downward through the soil and underlying rocks
dissolves and holds in solution many of the mineral
substances which it meets with (see p. 84), and that under
certain circumstances these substances may be redeposited
in the cracks and interstices of the rocks at a greater or
CHAP. TI.J CONSOLIDATION OF KOCKS.
393
less deptli from the surface. This process of infiltration
may go on till all the interspaces between the original
particles of a rock are filled up with crystalline or colloid
matter, and the whole mass is bound into a hard and
compact mass. This cement may be ferruginous, calcareous,
or siliceous.
In coloured sandstones the grains are generally found to
be coated with a film of peroxide of iron, which has been
formed in the manner explained on p. 86 ; this gives cha¬
racteristic red, brown, and yellow tints, according to the
thickness of the film, and it unites the grains together,
though it seldom completely fills the interstices between
them.
The cementing power of oxidized iron is seen when any
mass of the metal is allowed to rust in water ; a few horse¬
shoes or a bag of nails will suffice to agglutinate a
considerable mass of material. Dr. Mantell has described
a ferruginous conglomerate dredged up off Hastings,
consisting of glass beads, knives, and sand, the two former
being derived from a Dutch vessel which was stranded
there in the last century.
Calcareous infiltrations are also very frequent. Calca¬
reous sandstones have been described on p. 358, and the
manner in which the calcite crystals sometimes embody
the sand-grains was there noticed. A curious modification
of this structure occurs in the sands of Fontainebleau,
from which free and perfect rhomboids, consisting of fine
sand embedded in calcite, have been obtained. Many
limestones have been consolidated by the subsequent
infiltration of calcite (see p. 362), which has gathered the
fine paste or matrix of the rock into its crystalline struc¬
ture, and has produced a more or less crystalline lime¬
stone. At the same time, all the organic remains which
consisted of aragonite have been dissolved and replaced
by crystalline calcite. When a slice of such rock is ex¬
amined under the microscope, the calcitic matrix is seen
to be opaque or cloudy with the included calcareous mud,
while all the spaces which were not originally filled with
this mud, and all the replaced aragonite structures, appear
in clear transparent calcite.
Siliceous infiltrations have also been mentioned (p. 359) ;
394
STRUCTUKAL GEOLOGY.
[PAET II.
the Hertfordshire puddingstone is an excellent instance of
such consolidation, for the pebbles and sand-grains of
which it mainly consists are so firmly glued together by
the chalcedonic cement that a fracture breaks across them,
even dividing the sand-grains, which appear as clear
crystal-like specks on the fractured surface. The " grey
wethers " and " sarsen stones," so common on the chalk
downs of Berks and Wilts, are relics of similarly agglu¬
tinated sandstones. A fine-grained sand embodied in a
chalcedonic cement is often called a chert ; thus Mr.
A. Strahan describes ' a mass of " chert " in Flintshire over
300 feet thick, which passes laterally into a granular
sandstone with a siliceous cement, and ultimately into a
conglomerate or puddingstone.
4. Chemical Change.— In some cases the whole
material of a rock is altered from its original condition
into a different chemical substance, and this is sometimes
a cause of consolidation. The conversion of woody and
peaty matter into coal comes under this head, for though,
no doubt, it is partly due to simple pressure, and the
pressure may have assisted the chemical action, still the
gradual elimination of oxygen, and the condensation of
the solid carbon, must have tended to the consolidation of
the mass.
The alteration of limestone into dolomite by the filtration
of water containing magnesio chloride and siilphate, result¬
ing in the replacement of part of the hme by magnesia,
which combines with the remaining calcic carbonate to
form dolomite, is another instance of a chemical change
which causes consolidation by the consequent contraction
and crystallization of the rock.
5. Heat assists the processes of consolidation, which
have already been mentioned, and it has also a special
action of its own.
Sedimentary rocks may have come within the influence
of heat in three ways. They may have been buried so
deeply by other deposits that for a time they occupied a
low position in the earth's crust, where the temperature
was much greater than near the surface. As the tempera-
' " Mem. Geol. Survey," Explanation of Sheet 79, N.W.
CHAP. VI.]
CONSOLIDATION OF ROCKS.
395
ture at the bottom of Eose Bridge Colliery (2,450 feet) is
94° F. (see p. 12), that at the bottom of the Sperenberg
boring (4,040 feet) is 120°, and that of the water from a
boring near Pesth (about 3,000 feet deep) is 160°, it is
evident that the water in rocks which lie at 4,000 to 5,000^
feet below the surface must everywhere be very hot, and.
that at a depth of between 7,000 and 8,000 feet it must in
many places be as hot as boiling water is at the surface.
Now we have already stated that strata which were formed
at the surface have frequently been buried to a much
greater depth than 8,000 feet.
In volcanic districts the rocks are heated by the influence
of the rising lava, and may acquire a high temperature at
a comparatively small depth from the surface. In every
district, therefore, where volcanic rocks occur, we may
expect to find evidence of the effects of heat on stratified
rocks.
Lastly, heat may be developed locally by mechanical
force, such as that of lateral pressure, which has sometimes
been carried to the extent of crushing the rocks.
The hardening effect of dry heat is well known, and any
rock exposed to the contact of liquid lava must be baked
and indurated ; the phenomena of such contact-metamor-
phism will be described in a future chapter.
The passage of heated water through stratified rocks,
whether in ascending or descending currents, must have a
powerful effect in dissolving and rearranging the con¬
stituents of the rocks, and its continued action may lead
to a high degree of alteration and consolidation.
It will suffice in this connection to indicate some of the
changes which can be produced by this agency:—Hot
water attacks and dissolves mineral matter much more
readily than cold water, so that, as we often see in hot
springs and geysers, pure silica can be held in solution by
water which is below the boiling point, and is deposited as
the water cools. This process must frequently go on
beneath the surface, and the silica must be deposited in
the form of a siliceous cement in those portions of the
rocks where it loses heat.
The changes which can be effected without a very high
temperature are illustrated by the formation of many
396
STRUCTURAL GEOLOGY.
[PAET II.
minerals, including some hydrated silicates or zeolites, in
the interstices of the bricks of a Roman bath at Plombières
in France by the waters of a warm spring. The minerals
were formed by the action of the water on the materials of
the bricks, and its temperature is only 122° F., or about
that of the water at Bath in England.'
Not only can moderately warm water effect such results,
but the very solutions which it takes up at ordinary
temperatures increase its capacity for dissolving other
minerals, and if such water percolates downward till it
acquires a high temperature, none of the ordinary con¬
stituents of rocks could withstand its potency. Dr. Sterry
Hunt has shown that water containing alkaline carbonates
will, at a temperature of not more than 212° F., rapidly
dissolve silica, and generate various silicates. Again, it
was proved by Daubree that in the presence of dissolved
alkaline silicates at a temperature above 700° F., various
siliceous minerals, such as quartz, felspar, augite, could be
formed and crystallized out.^
COKCEETIONS.
It win be convenient in this connection to give some
account of the different kinds of nodules which occur so
frequently in stratified rocks, and are known by the name
of concretions. As this term is often used in a loose and
ill-defined sense, we shall here restrict it to such lumps or
nodules of mineral matter as have a different chemical
composition from that of the rock-material which encloses
them ; those nodules which are merely spheroidal lumps of
the rock-mass, developed during the process of consolida¬
tion, are therefore excluded from the present category.
These latter we regard as structural developments pro¬
duced by physical forces, while the former have been
produced by the operation of chemical forces. This che¬
mical action may have operated during the deposition of
the bed in which the concretions are found, or during its
consolidation, and it is a mistake to suppose that they
' " Quart. Joum. Geol. Soc.," 1878, p. 73.
® " Bull. Soc. Geol. de France," torn. xv. p. 103.
CHAP. VI.J
CONSOLIDATION OF ROCKS.
397
have been entirely formed by a process of segregation
after the consolidation of the surrounding material.
In most cases it is evident that the concretions must have
been formed while the enclosing deposit was sufficiently
fresh and plastic to allow of free molecular movement
within its mass ; it has also been proved that many such
nodules owe their origin to the decomposition of organic
matter, and in these cases the concretions must have begun
to form while the rock-mass was still permeated with the
water in which it was deposited, and before it had been
covered up by any very great thickness of subsequently-
deposited material. Organic bodies decomposing on the
muddy bottom of a lake or sea evolve carbonic acid, car¬
bonate of ammonia, and other chemical substances, which
react on the substances dissolved in the water, and cause
the precipitation of various mineral compounds. Other
causes may assist in the production of concretionary
nodules, and the whole subject requires farther study and
investigation. "We now proceed to notice the different
kinds of concretions which occur among aqueous rocks.
In Sandstones concretionary nodules are generally
either calcareous or ferruginous. In some sandstones large
spheroidal masses of calcareous stone occur, which appear
to be portions of the sandstone bound together by car¬
bonate of lime. These are called doggers in the north of
England ; they often abound in fossil remains, while the
loose sand outside does not contain any. Other sandstones
contain concretions of iron pyrites, varying in size from
mere grains up to masses as large as the fist.
In Clays and Marls. There are few large deposits of
clay which do not contain concretions of some kind.
Sectaria consist chiefly of carbonate of lime with some
carbonate of iron, and occur in many clays, forming large
nodules which sometimes attain a diameter of 2 to 3 feet,
or even more. In true septaria the interior is split up by
numerous cracks, which are generally filled with calcite,
forming divisions or septa, but similar concretions which
are entirely solid often occur in clays.
Gornstones.—Beds of comstone have been mentioned on
p. 358, but these often pass into layers of separate con¬
cretions, which may be called cornstone nodules.
398
STRUCTURAL GEOLOGY.
[PAET II.
Clay-ironstone nodules are of similar construction, but
consist chiefly of carbonate of iron mixed with more or less
clay and carbonate of lime. When broken open the smaller
nodules frequently disclose a fossil shell or leaf which has
■evidently formed the nucleus round which the ferruginous
matter has accumulated; they often have a septarian
structure.
Phosphatic nodules occur in clays and calcareous shales,
and are composed of phosphate and carbonate of lime in
variable proportion. Those from the Gault and Cambridge
greensand contain from 56 to 60 per cent, of the phosphate,
with 10 to 12 per cent, of the carbonate, but the Suffolk
nodules (derived from the London Clay) are less rich in
phosphate with more carbonate of lime, and a considerable
amount of iron. They are commonly but erroneously called
Coprolites.
Marcasite nodules, or balls consisting of that variety of
iron pyrites called Marcasite, are also abundant in many
clays, especially where organic remains are abundant, and
frequently enclose some fossil shell or plant.
In Limestones the concretions are either siliceous,
ferruginous, or phosphatic.
Flint and Chert are weil-known substances, the com¬
position of which has already been described ; both occur
as nodules of irregular, and often eccentric form.
The origin of the flints in the chalk, and of the chert
nodules in other limestones, was long a matter of doubt,
but recent researches have gone far to explain the process.
Dr. Bowerbank was the first to suggest that sponges had
been much concerned in the formation of flints ; and re¬
cently Professor W. J. Solías has made this more certain,
and has given the following explanation of their origin.
He thinks that the siliceous spicules of numerous sponges
gradually accumulating in the chalk ooze, and in presence
of sea-water under great pressure, would enter into solu¬
tion as organic silica ; that where the sponge growth was
very thick, it contributed so much silica to this solution
as to render it concentrated enough for the deposition of
mineral silica, which replaced a certain amount of the
calcareous ooze. The calcareous material thus became
siliceous chalk, and ultimately a further deposition of silica
CHAP. VI.]
CONSOLIDATION OF ROCKS.
399
replacing the remainder of the calcic carbonate converted
the siliceous chalk into the dark flint as we now find it.
This view receives confirmation from the discovery of
siliceous chalk with imperfectly formed flints in Wilt¬
shire.'
Marcasite nodules also occur in some limestones, gene¬
rally in the form of rounded lumps or balls, which have a
radiated crystalline structure internally. Many of those
in the Lower Chalk of Folkestone are irregular in form,
and seem to have been formed by the pyritization of the
siliceous sponges known as Ventriculites. The connection
of iron-pyrites with decaying organic matter is so per¬
sistent that we may safely attribute all such pyritous
nodules to the decomposition of organic bodies.
Phosphatic nodules are occasionally found in shaly lime¬
stones, as in the Bala limestone of North Wales. Nodules
containing more or less phosphate of lime also occur at
several horizons in the chalk ; they seem to be generally
lumps of chalky mud which have been derived from an
older deposit of Chalk, and partially consolidated before
being washed into the bed where they are now found ; they
are generally coated with a green chloritic or glauconite
substance. Those in the Chalk-rock contain 5 or 6 per
cent, of the phosphate, those in the Lower Chalk 15 to 20
per cent, and those from the Chloritic Marl from 35 to 50
per cent.
' See "Quart. Journ. Geol. Soc.," vol. xlv. p. 410.
CHAPTEE VII.
JOINTS.
IN Chapter Y. stratified rocks were described as consisting
of layers or strata separated by planes of division
which result from the manner in which the strata were
accumulated ; but all rocks, whether stratified or unstra-
tified, exhibit other features which have been produced
during the process of their consolidation.
There are few exposures of stratified rocks which do
not exhibit certain other planes of division besides those of
lamination or stratification. These are the joints, or long
cracks which cut across the bedding planes and separate
each bed into blocks of various shapes. It is obvious that
in stratified rocks there must be at least two sets of joint
planes besides the planes of stratification, in order to cut
up the beds into blocks. In igneous or unstratified rocks
it is equally obvious that in order to form squarish blocks
we must have at least three sets of joints crossing each
other, each set more or less nearly at right angles to the
other two.
Origin of Joint Structures.—Everyone who has
examined the muddy bottom of a dried-up pond or lake is
aware that, as the water evaporates and the mud dries,
long and deep cracks are formed which traverse the dry
mud or clay in various directions, and cut it into polygonal
blocks. The width and depth of these cracks depend
partly upon the extent and thickness of the mud.
In the same way molten rock shrinks and cracks in
cooling. Attempts have been made to utilize the slags
from iron-furnaces by running them into moulds, but the
CHAP. TII.j
JOINTS,
401
quadrangular blocks so obtained proved to be useless for
building purposes because they cracked and crumbled into
small cuboidal fragments in consequence of the numerous
joints developed during the process of cooling and solidi¬
fication. Differences in the texture of rocks will naturally
cause differences in their manner of cracking, and so
different forms of jointing will be produced. The number
of joint-planes will also partly depend upon the thickness
of the bed or mass. Some beds wül shrink more than
others, and some masses of igneous rock must have cooled
more rapidly than others.
Mr. Jukes has remarked that it does not follow that
all the joints in any mass of rock should be formed at any
one time. " The consolidation of the mass may take place
slowly and gradually, and successive sets of joints be pro¬
duced in it at different times during that process. A rock,
moreover, may be, at some subsequent period, placed
tmder circumstances calculated to produce a greater degree
of consolidation, and a fresh set of joints may be produced
in it from that cause.
" The small or short joints confined to individual beds
of stratified rocks may have been those first formed on the
original consohdation of the one bed before the next was
deposited on it, those joints being then, perhaps, quite im¬
perceptible divisional planes with no interspace between
the blocks. Whole sets of beds may have subsequently
been subject to one, two, or more actions of consolidation,
which may have produced larger joints traversing the
whole mass. Still more extensive joints may have been
formed subsequently by the mechanical agency of the up¬
heaving forces acting on the crust of the globe." ^
M. Daubree has lately obtained some curious results by
submitting plates of various substances to the effects of
torsion, and finding that he could by this means produce
sets of cross fractures similar to ordinary joints, he boldly
ascribes all joints to the action of force similarly applied.*
Mr. J. G-. Goodchild,' however, has pointed out that joints
are of various kinds, and are not likely to have all been
' Jukes' "Manual of Geology," second edition, p. 218,
^ " Geologie Experimental," vol. i. p. 300 ei seg"
' " Geol. Mag., Dec. 2, vol. x. p. 397.
n D
402
STRUCTURAL GEOLOGY.
[PAET II.
formed in the same way. Some are clearly shrinkage
cracks, while the larger " rift-joints " he regards as pro¬
duced by the unequal pressures and strains exerted upon
rock-masses during movements of subsidence and up¬
heaval.'
Cuboidal Jointing.—Any mass of stratified rock
which is traversed by two sets of joints that are roughly
at right angles to one another will fall into cvboidal or
squarish blocks. This may be seen in almost any stone-
quarry, and fig. 96 may be taken to represent a part of such
a quarry. The rock-faces, one set of which receive the
Fig. 96.
light, while the other set are in shadow, are the surfaces
formed by the two sets of joints cutting the planes of
stratification at right angles, and also cutting each other
at right angles, and thus making the square corners.
A good-sized clean lump of coal taken from the coal¬
scuttle forms an excellent model for the exhibition of lami¬
nation and joint-planes. If the student wUl place such a
lump on the table before him, he will perceive that the
mass is divided by three regular and distinct sets of
division planes. " The coal splits most readily along the
' See also G. K. Gilbert, " Amer. Journ. of Science," ser. 3, vol.
xxiv. p. 50.
CHAP. VII.J
JOINTS.
403
lines of lamination, and the surfaces thus exposed along
the tops and bottoms of the lumps are generally dull and
earthy, and readily soil the fingers. At right angles to
these surfaces others may be observed, which are generally
bright and shining, and if the coal be freshly broken, these
surfaces will soil the fingers much less than those on the
top or bottom of the lump. He will see that there is one
set of smooth, vertical surfaces (or joint planes), along
which there occur the cleanest, largest, and most even sides
to the block, the vertical surfaces at right angles to that
set being shorter, rougher, and more irregular. The first
large, smooth, vertical surfaces are known by the name of
the face, the slyne, or the cleat, of the coal in different
districts,—the more interrupted set being spoken of some¬
times as the end of the coal. " '
In a series of'stratified rocks each bed generally has its
own system of joints, which are close and regular and do
not penetrate the beds above and below. There are always
others, however, which are common to several successive
beds, and seem to have been formed simultaneously, though
they "often change their direction a little in passing from
one bed to another. Lastly, there are some still larger and
longer planes of division, which cut through many succes¬
sive beds, and maintain the same direction both vertically
and laterally for very long distances; these are called
the master-joints. The master-joints sometimes form a
more conspicuous feature in the rock than the planes of
stratification do, and are occasionally found to cut across
whole mountains in regular parallel lines. These large
joints are often open fissures with a space of an inch or
more between the walls, and in limestones this space has
frequently been widened still more by the action of per¬
colating water. Such joints are always well developed in
limestone districts (see the Frontispiece, and also p. 86,
and fig. 28).
Surface Exhibition of Joints.—Although the general
characters and vertical course of joints may be readily ob¬
served in almost any stone-quarry, there are not such
frequent opportunities of studying their horizontal exten-
\ Jukes' " Manual oí Geology,'' second edition, p. 212
404
STRUCTURAL GEOLOGY.
[part ii.
BÍon or surface exhibition over large areas. Mr. Jukes
found that the newly-formed beds of stone which occur on
some of the coral islands off North-east Australia were
already divided by a system of joint-planes. He says :—
"I often observed several beds of stone resting on each
other, each more than a foot thick, inclined at an angle of
8° or 10° ; that is to say, at the same angle as the slope of
the beach or bank of sand on which they rested. They
had, to all appearance, been consolidated in this position.
The joints which traversed them, although often uneven
and jagged, ran in straight parallel lines over spaces some¬
times of 200 yards, or as far as they could be seen, their
planes being generally at right angles to those of the beds,
one set of joints running along the greatest linear exten¬
sion of the mass (strike-joints), and the other set directly
across the former, and in the same direction as the inclina¬
tion of the mass (dip-joints). The directions of these two
sets of joints seemed to depend in these cases on the direc¬
tions of the principal bounding surfaces or edges of the
mass." '
The surface exposure of a hard rock like limestone is
sometimes so bare of soil as to present an excellent field
for observing the horizontal extension of joint-lines, and
perhaps no district offers better facilities for such a study
than the barony of Burren in county Clare, Ireland. Hills
of limestone here rise more than 1,000 feet above the sea,
the stratification being almost horizontal, the hill summits
and the terraces that sweep round their sides showing
broad fioors of bare rock over the whole country. The
joints, which are very numerous and very regular, have
been widened by the rain, so as to form superficial crevices,
sometimes several inches in width and several feet in
depth. The floors of limestone are cut by them into a
number of separate blocks of quadrangular and triangular
forms. The late Mr. F. J. Foot, of the Geological Survey,
who examined this district, has given a detailed account of
thesejjoints in the explanations of the maps 114, 122, and
123, of the Geological Survey of Ireland. He found that
there were three distinct sets of joints, besides other small
* Jukes' " Manual of Geology," second edition, p. 217.
CHAP. VII.]
JOINTS.
405
and irregular cracks. Two of these sets of joint-planes
were nearly parallel to one another and ran nearly north
and south, one a little E. of N., the other a httle W. of
N., so as to produce large wedge-shaped blocks several feet
long, ending in angles sometimes as sharp às 5°. These
master-joints were crossed by others running nearly east
änd west, not in straight, but in gently curved serpentine
lines. The straight north and south joints sometimes
stopped suddenly at one of these cross joints, and it was
observable that the neighbourhood of one joint-plane, or
the space between two adjoining planes, exhibited a number
of closely adjacent parallel joints, not more than an inch
or so apart, which split the beds across into vertical slabs
(see p. 86, and fig. 22.)
Scale about 12 feet to an inch.
The disposition of joints may also be studied on rocky
shores, where the surface of one hard bed is sometimes
bared for a considerable extent between tide-marks.
Horizontal Jointing.—Besides the vertical joints
which are so conspicuous in all stratified rocks, a series of
horizontal joints is frequently developed. In many cases
it is difficult to distinguish these from the planes of
bedding, and probably they are most strongly developed
in massive homogeneous rocks which do not exhibit frequent
or distinct planes of stratification. They are specially
noticeable in the more compact and marly varieties of
chalk where the usual planes of bedding are entirely ob¬
scured or replaced by a set of irregular and curvilinear
joint-planes, which bear some resemblance to those of
actual curvilinear bedding produced by current action (see
406
STRUCTURAL GEOLOGY.
[part ir.
p. 386). Fig. 97 is taken from a sketch in a chalk quarry
near Cambridge, where this structure is well exhibited.
A somewhat similar kind of jointing has sometimes
been produced in igneous rocks, and has been called " Cur-
vitabular structure" by Professor Bonney, who has described
an instance in a part of the basalt of the Plateau de la.
Prudelle, Auvergne. " The mass," he says, " is traversed
by a series of more or less horizontal curving joints, with
their convexities upward; so that the whole is divided
into a series of plano-convex, meniscoid, or concavo-convex
blocks, in length perhaps about 2 to 4 feet, and in thickness-
from 4 to 12 inches. This structure continues with but
slight indications of any tendency to vertical master-joints
for some 35 feet." ^
Granite and other igneous rocks often exhibit a much
Fig. 98. Joints in Granite near Penrhyn.
q q, Open quarries. r r, Unquarried rock.
more regular series of horizontal joints, which together
with the vertical joints, split up the mass into a succession
of tabular blocks. Such joints are especially well developed
in the granites of Cornwall, where they are called bedding-
joints by the miners. It is a noteworthy fact that these
bedding-joints are not truly horizontal, but run in broad
curves, which closely correspond with the curvatures of the
surface of the ground. The quarries are generally opened
on the slopes or summits of hills, and it is stated by Mr,
G. F. Harris that in every case the run of these joints
aj^proximately follows the curve of the hiU.^ Fig. 98 is
reproduced from one given by him to illustrate this, and
shows the directions of the two main sets of joints in the
^ " Quart. Journ. Geol. Soc.," vol. xxxii. p. 146.
' " Granites and Granite Industries," 1889, p. 104.
CHAP. VII.J
JOINTS.
407
quarries on Carnsew Hill, near Penrhyn. The " bedding-
joints " are crossed by nearly vertical master-joints, and
again at nearly right angles by another set of steeply
inclined joints, so that the rock-mass breaks into nearly
cubical blocks. When these blocks are exposed to atmo¬
spheric disintegration the comers and edges crumble away,
the joints are widened, and the result is a pile of separate
blocks, resting loosely one on the other. In granite
districts these piles are called torrs or cheese wrings ; the
logging-stones or rocking-stones are also due to the develop¬
ment of a similar structure (see p. 100).
As regards the origin of this horizontal jointing, the
tabular structure in granite and other igneous rocks may,
in some cases, be due to the tensions caused by alternate
expansion and contraction under diurnal variations of tem¬
perature ; for rapid contraction at night after expansion by
the sun's beat during the day is likely to produce normal
as well as horizontal strains in the rock-mass.
The curvilinear planes in chalk may perhaps be due to
an analogous process, for chalk must be particularly liable
to such changes of dimension in consequence of its capacity
for holding water. The drawing up and evaporation of
this water by the sun during dry weather must cause rapid
desiccation and contraction. Near the surface the chalk is
broken up into small tabular fragments, but lower down
the result may be different. The strains in a horizontal
direction will be relieved by vertical joints, but there must
also be strains in a vertical direction, and these can only
be relieved by cracks which are more or less horizontal or
parallel to the surface of the ground. Once formed they
would always remain as divisional planes, and the subse¬
quent passage of water along them would only tend to
keep them open.
Prismatic Jointing.—Unstratified rocks cannot fall
into cubical blocks unless the vertical joints are crossed by
another set of horizontal joints. Without these it is clear
that the vertical joints will split the mass into long
columns or prisms, the shape of which will depend on the
number of intersecting joint-planes. This form of jointing
has been termed prismatic, as tending to the formation of
prisms, and it is only from rocks which are thus prismati-
408
STRUCTURAL GEOLOGY.
[part ii.
cally pointed that the great monolithic pillars used for
obelisks and other monuments can be obtained.
Granite, syenite, and other igneous rocks which occur
in thick masses, are usually traversed by two sets of joints
more or less at right angles to one another, so that the
mass is split up into huge oblong blocks or square-cornered
prisms many yards in length.
Fig. 99 is an instance of such joints in dolerite at North
Queensferry, Pifeshire, sketched by Professor T. G. Bonney,
Fig. 99. Joints in Dolerite.
and copied with his permission ; some of the great tabular
blocks formed by the joints were unbroken from the top
to the floor of the quarry, a height of 50 or 60 feet, and
stretched still farther downwards, while their faces were
as smooth as a plastered wall.
In the granite quarries at Syene (Upper Egypt), whence
the ancient Egyptians obtained the material for so many
of their monuments, a monolithic block may be seen
partially removed from its site, bnt cracked in the process.
If the continuity of the prisms is broken by the occurrence
of a third set of horizontal joints, the rock separates into
rude square-shaped prisms or columns each composed of
a number of quadrangular blocks (as mentioned above) ;
CHAP. VII. J
JOINTS.
409
this structure is exhibited in some of the granite cliffs
between Penzance and the Land's End.
The more compact and even-grained igneous rocks, espe¬
cially those forming dykes and lava-flows, often exhibit a
more regular kind of prismatic jointing. This is generally
termed columnar structure, because the joints are so
arranged as to divide the mass into a series of close-set
polygonal columns. These columns may be vertical, as in
fig. 100, or horizontal, as in fig. 101, or they may be variously
bent and curved. They vary in length from 5 to upwards
of 100 feet, and in diameter from 6 inches to several feet.
The Giant's Causeway in county Antrim, and Fingal's
Fig. 100.
Fig. 101.
Cave in the Island of Staffa, are well-known examples of
this columnar structure.
The Isle of Staffa consists entirely of basalt, the upper
part being massive, and the lower part is beautifully
columnar. It is about two miles in circumference, and
forms a table-land of an irregular surface, the greatest
elevation of which is about 12Ô feet. It is intersected by
a deep gorge which divides the higher and best-known
portion from the other part of the island. At the highest
tides, the columns which form the south-western cliffs
appear to terminate abruptly in the water ; but the re¬
tiring tide exposes a causeway of broken columns at their
base.' Fingal's Cave is excavated in this columnar basalt,
' See Mantell's " Wonders of Geology," p. 891, from which fig.
102 is borrowed.
410
STRUCTURAL GEOLOGY.
[part ii.
and fig. 102 gives some idea of the appearance of its
interior. In some places, and especially at Clamshell Cove,
the columns are curved, and resemble the ribs of some
huge ship's hull.
The columns are generally more or less regularly hexa¬
gonal, and Mr. Mallet has shown that on the mathe¬
matical principle of least action, a mass of molten rock
will naturally tend to break into hexagonal prisms at
right angles to the surfaces of cooling. A surface can be
divided up completely into the three following equilateral
Fig. 102. View of Fingal's Cave.
figures,—equilateral triangles, squares, and hexagons. Of
these the last, for a given area, has the smallest perimeter ;
hence, when such a figure is forming by cracking, there
will be the least resistance to rupture from the cohesion of
the particles of the mass. Again, in the case of the last,
Mr. Mallet shows that, of the contractile force causing the
rupture, the component which acts perpendicularly to the
sides, and is therefore the efS.cient part, is the greatest.
Hence the amount of work which has to be done in order
to fonn a hexagonal column out of a contracting mass is
less than a square or triangular one.
chap. vii.]
JOINTS.
411
Mr. Fisher has kindly supplied me with another demon¬
stration of this, which is perhaps more easily understood.
In a homogeneous rock like basalt the contractile force-
seems to be exerted so evenly throughout the mass, that,
we may assume it to tend toward a set of equidistant
points. Such equidistant points must be at the comers of
equilateral triangles. Let a b c be such points, and
suppose the rock-substance to be drawing away from each
point equally, this substance will part at points which are-
midway between a and b, a and c, b and c. The cracks
so generated will run in straight lines till they meet. Let
d be another point equi¬
distant from b and c, and A
suppose similar cracks to be
formed between b and d
and between c and d, then
it is easy to see that, when
a complete system of such
cracks is produced, all ex¬
tending till they meet one
another, the spaces enclosed
by them will be hexagons.
The cracks thus commenced
will strike down into the
cooling mass, and will re¬
sult in the formation of
regular hexagonal columns.
The columns are straight and regular in proportion as
the cooling proceeded uniformly from a plane surface ; but
if the cooling surface was convex, the columns must strike
inwards towards a centre ; while, if it was concave, they
would radiate outwards in different directions. Again, the
opening of fissures within the mass would provide other
cooling surfaces from which independent sets of columns
would be started. Thus it frequently happens that
in the upper part of extruded bosses or streams of
lava, the columns are inclined or curved in various direc¬
tions, while the lower part of the same mass is regu¬
larly divided into straight and vertical pillars ; the two
portions being separated by a more or less horizontal plane
of tension.
412
STRUCTURAL GEOLOGY. [PAET II.
Again, in dykes of igneous rock the columns are always
at right angles to the bounding walls, and have evidently
started from these surfaces where the cooling and consequent
consolidation would necessarily commence, as in fig. 101.
Moreover, it often happens that the columns are not con¬
tinuous across the dyke, but are separated into two sets by
a medial plane. In this case it is clear that the columns
originating on either side of the dyke struck inwards, and
met along the medial line, but the axes of the columns
not being in the same line the two sets of columns could
not coalesce into uniform bars.
Articulated Columns and Spheroidal Structure.
—Columnar lavas sometimes exhibit a still more curious
and complicated structure, occasioned by transverse articu¬
lations or planes of division which tiaverse the columns at
nearly equal distances so as to split them up into a series
of separate hexagonal blocks. The surfaces of these trans¬
verse cracks are sometimes flat, but more often curved,
and sometimes they fit into one another with a kind of ball-
and-socket arrangement. Professor Bonney has shown
that all these jointed, tabular, and cup-and-ball struc¬
tures are referable to the same cause, viz., contraction
while cooling ; and in one case he has noted a sort of tran¬
sition from a regular jointed column to a broad prism with
slightly curved walls and a kind of curvitabular cross-
jointing (see fig. 104).
In other cases a spheroidal structure has been developed,
the whole mass of rock separating into large balls which
exfoliate undér the action of the weather into regular con¬
centric layers or coats, bke those of a large onion. It was
formerly supposed that this spheroidal structure was sim¬
ply a mode of weathering. It is true that cuboidal blocks
ef rock, such as those of granite, have a tendency to weather
into rough spheroids, and further decomposition might
sometimes produce a kind of concentric exfoliation ; but
this is not sufficient to explain all the instances of
spheroidal structure that have been observed.
In the first place, a similar structure is occasionally
■developed in rocks which are not jointed so as to
split into cuboidal blocks ; an excellent example of this
in bedded shale has been described and figured by Mr.
CHAP. TH.]
JOINTS.
4ia
Jukes.^ Again, spheroids may he found inside a single un-
jointed prism of columnar rock, instances of which are
described by Professor Bonney,^ and one is reproduced,
by permission in fig. 105. It was sketched from a mass
of columnar basalt near Le Puy in Auvergne, and exhibits
several spheroids, one above another, without any dividing
cross-joints. " Similar instances of spheroids disconnected
from the main joints may he seen in a quarry called Turner's
Pit, in the Eowley-Eegis basalt (Staffordshire), where,
also, in some cases, the nuclei of the spheroids are further
subdivided, so that imperfect spheroids are enclosed
by a (larger) spheroidal shell like the twin kernels of a
Such instances seem to indicate, as Professor Eonney
remarks, that the divisional surfaces of the spheroids and
the columns are, to a certain extent, independent of one
another, namely, that the cracks opened from without in-
* " Manual of Geology," third edition, p. 311 ; " School Manual,"
fifth edition, 1890, p. 137.
2 "Quart. Journ. Geol. Soc.,"vol. xxxii. p. 151.
Fig. 105. Spheroids in Co¬
lumnar Basalt near Le
Puy, Auvergne.
Fig. 104. Structure of Columns
in Lava at Koyat.
nut."
414
STRUCTURAL GEOLOGY.
[part ii.
wards to the less solidified part, and the spheroids, at the
same time, formed within this more plastic part of the
■column, the two surfaces of division running together at
last so as to complete the separation.
The precise manner in which these spheroids are deve¬
loped is not very easy of explanation, but if it be assumed
that contraction is taking place uniformly throughout the
mass of the coohng material, then spheres may be formed
in this mass on the same principle by which hexagons are
formed on a plane surface ; a sphere being the solid of
least superficies for a given volume, and being the figure
where the contractile force acts everywhere perpendicularly
to the circumference. If the joints simultaneously pro-
■duced are few and far apart, the spheres will remain sphe¬
roidal ; but if hexagonal columns are being produced at
the same time, the spheroids necessarily come to have a
hexagonal circumference, and are regularly superimposed
one on another. Whether the top and bottom of each
block is flat, convex, or concave, depends probably on
differences in the strains or tensions within the mass.
Art of Quarrying.—Without natural joints thequarry-
ing of stratified rocks would be very difficult, and that of
igneous rocks would be almost impossible, for each block
would have to be cut out on every side from the rest of the
solid mass. The whole art of quan-ying consists in taking
advantage of the natural division of the rock by the planes
of jointing and bedding, where the latter exist. The shape
of the quarry will therefore depend on the direction of
the master-joints which traverse the stone. One set of
these joints will form what is called the face or hack of
the quarry or the boundary wall towards which the men
are at any time working; while the other set of joints,
at right angles to these, are the planes along which they
work ; these are called the ends, or sometimes the cutters
of the stone.
In mining operations where adits and galleries are driven
into the rock, it is still more necessary to observe the direc¬
tion of the joint-planes, for by these natural lines of divi¬
sion will the direction of the passages be regulated. This
is particularly the case in coal-mining, for, as already stated,
beds of coal exhibit not only the ordinary system of distant
CHAP. VII.J
JOINTS.
415
joint-planes, but also a system of minor joints wbich retain
their parallelism over very large areas ; the main galleries
of a coal-mine, therefore, are always driven along the face
of the coal, while the cross galleries run along the end of
the coal. To attempt to cut galleries across that direction
in which the coal will naturally split into blocks would
obviously be a much more difficult and expensive task than
to take advantage of this structure.
CHAPTEE VIII.
INCLINATION AND FLEXUEE OF BEDS.
E could not continue our observations upon stratified
rocks very long without perceiving that their beds
are not always horizontal, but are, on the contrary,
generally inclined to the horizon at a greater or less
We have already seen that in certain cases beds of
stratified rock may have been originally deposited on a
considerable slope. Such beds, however, can only be of
very limited extent, and we can hardly imagine a succes¬
sion of parallel beds to have been deposited with exactly
the same slope ; neither is it possible for materials finer
than coarse sand to rest on a slope having a greater incli¬
nation than 35°. Whenever, then, we find a set of inclined
beds maintaining an equal thickness and approximate
parallelism over a wide extent of ground, we may feel
quite sure that when first formed those beds must have
been practically horizontal, and that their present inclina¬
tion is the result of subsequent movement. They must
have been tilted, either by being lifted up at one end or
depressed at the other, and in many cases the beds have
been subjected to such powerful and long-continued move¬
ments that they are tilted up at very high angles, and
in some cases are set on edge in an absolutely vertical
position.
Dip and Strike.—The inclination thus given to beds
of rock is called their dip ; its amount is expressed by the
number of degrees contained in the angle between the
plane of the beds and the plane of the horizon, and the
angle.
CHAP. VIII.J INCLINATION AND FLEXURE OF BEDS. 417
instrument by which this angle is measured is called a
clinometer.
Clinometers are made in various ways, but the simplest
form is that shown in fig. 106. It consists of a thin piece
of wood on which is engraved a semicircle graduated into
degrees, with a pendulum or plumb-line fixed in the centre
of the arc. When held horizontally the plumb-line wiU be
at 0 ; when inclined to either side it vdll indicate the angle
which the upper edge of the wood makes with the plane of
the horizon. If, therefore, the clinometer is held so that its
upper edge is parallel to the lines of bedding, the amount
or angle of dip is easily read off.
The appearance of an inclined bed at the surface is
called its outcrop or basset, and the line along which the
Fig. 106. A Clinometer.
upper surface of a bed rises from beneath another is
called the line of outcrop. The line of its lower surface is
sometimes called its boundary, but this boundary line must
necessarily coincide with the line of outcrop of the sub¬
jacent bed.
The line at right angles to the plane of the dip is called
the strike of the bed. The line of strike is in fact the inter¬
section of the plane of the horizon with the plane of the
dipping stratum. The strike is always in the plane of the
horizon, while the outcrop may be diverted by any irregu¬
larities in the surface of the ground ; hence the line of
strike wül only coincide with the line of outcrop when the
surface is level and horizontal.
E E
418
STRUCTURAL GEOLOGY. [PAET II.
Sir Charles Lyell compares dip and strike to a row of
houses running east and west, " the long ridge of the roof
representing the strike of the stratum of slates, which dip
on one side to the north, and on the other to the south."
The line of strike, however, is not always a straight line,
because any change in the direction of the dip will
produce a corresponding change in the direction of the
strike.
If we know the direction in which a bed dips, we know
also the exact bearing of its strike, but if we only ascertain
the strike, we cannot also infer the direction of its dip,
because it may incline to either side of the line of strike.
In geological surveying, therefore, it is most important to
observe the dip of aU stratified rocks, and to note its direc¬
tion accurately ; it is also important to know its amount,
or at any rate to ascertain the average angle at which any
group of rocks is inclined.
Geological Map and Section.—Any exposure of
rock at the surface, from which the nature and relative
position of the beds can be ascertained, is called by geo¬
logists a section. The exposure may be an artificial open¬
ing, such as a quarry, mine, or railway-cutting, or it may
be a natural section, such as the bank of a brook or the
face of a cliff. These sections are what a geologist first
seeks for and studies when he wishes to learn the subter¬
ranean structure of a district. By piecing together the in¬
formation obtained from such isolated exposures he is able
to construct a geological map of the district in which he is
placed, and also to draw a continuous section through any
part of it representing the arrangement of the rock-beds
as they would appear if some giant hand were to make a
vertical cut or trench through the country from the surface
to the " sea-level."
Fig. 107 is a diagrammatic view of the Isle of Wight,
illustrative of the appearance it would present if all the
surface soil was removed, and the landslips of the Under-
cliff were cleared away, so as to expose the actual outcrops
of the different beds over the southern half of the island.
Moreover, a huge slice is supposed to have been cut out of
the centre, so as to exhibit a vertical section through the
island from north to south.
A'
Fig. 107. Diagram of the Geological Structure of the Isle of Wight.
Cj Chalk, /, Upper greensand. g, Gault. gs^ Lower greensand. w, Wealden.
420
STRUCTURAL GEOLOGY.
[part ii.
The beds which compose the southern half of the Isle
of Wight have been divided as follows :—
5. Chalk at the top.
4. Sand and sandstone = Upper greensand
3. Blue clay = Gault.
2. Sands and sandstones = Lower greensand.
1. Clays and sandstones =Wealden.
It is seen that the beds composing these groups are nearly
horizontal in the foreground, but that as they are traced
northward toward the centre of the island they begin to
dip, the upper surface of each division being brought down
to the level of the sea, and the topmost beds of the chalk
passing beneath those which form the northern part of the
island, and are named Tertiary.
Fig. 107 is the representation of a geological model in
relief, but a geological map is only an ordinary map
coloured so as to indicate the outcrops of the principal
rock-groups. To construct such a map we proceed as
follows :—Taking a good map of the district we wish to
examine, together with a clinometer and a pocket com¬
pass, we search for all the natural and artificial sections.
If the district be near the coast, the sea-cliffs will probably
afford the best sections, and it will be desirable to ascer¬
tain the succession which they exhibit before attempting to
map the country inland.
We wiU suppose that along a certain part of the coast
we found a regular succession of beds, steadily dipping one
under another in a certain direction : the rocky scar at low
tide showing us the exact direction of their strike, the cliff
section enabling us to measure the exact angle of their dip.
Suppose, too, that the beds fell into five natural groups—
1, clays and ironstones ; 2, shales ; 3, sandstones ; 4,
shales ; 6, limestones ; each set of beds having some
peculiar characters of its own by which we should readily
recognize them again.
When, therefore, we came to trace the beds inland and
found a quarry where the sandstones (3) were exposed,
we should naturally expect to discover indications of the
shales above and below them somewhere on either side of
the quarry. If further search in one direction disclosed
CHAP. VIII.J INCLINATION AND FLEXURE OF BEDS. 421
crags of the limestone (5) on a hill-side, and in the oppo¬
site direction a brickyard in clays similar to those of group
1, we should feel sure that the structure of the country in¬
land was the same as that of the shore which we first
examined.
It is in this way, by gaining a knowledge of the true
succession of a series of beds where they are plainly ex¬
posed and then following them up across the country,
searching for all the small and partial exposures that
occur in the banks of brooks, rivers, and valleys, in
quarries and excavations of aU kinds, and thus carrying
on the line of outcrop from one place to another, that a
geological map is constructed. The spaces occupied by the
several rock-groups on the map are then coloured so as to
be easily distinguished. Por further information on the
methods of geological surveying we must refer the student
0
Fig. 108. Measurement of Depth and Thickness.
to one of the following works—Geikie's " Outlines of Field
Geology," or Penning's " Textbook of Field Geology."
As soon as the mapping of any particular district has
been completed, a geological section can be drawn across it
so as to illustrate its subterranean structure. To do this
we must first ascertain the outline of the ground along the
line of section ; for this purpose the sea-level is taken as a
datum or base-line, and some levelling instrument is used
by means of which the differences of level can be measured.
This process is also explained in the treatises already
referred to.
In fig. 108 the line a f may be taken as representing the
outline of the ground along the line selected, and the beds
represented are those we have supposed to exist in the
imaginary cliff-section. Knowing from the completed map
422
STRUCTURAL GEOLOGY.
[part ii.
the exact distance between the points where the upper and
lower boundary lines of each formation cut the line of sec¬
tion, we can draw lines from these points with the observed
inclination, in this case at 10° to the E.
When such a section has been constructed and drawn on
a definite scale, the total thickness of the beds exposed at
the surface can easily be calculated. Thus supposing the
section, fig. 108, to be drawn on the scale of six inches to
one mile, the thickness of the beds which come to the sur¬
face between the points b and c is shown by the line c o
drawn at right angles to their dip ; the length of this as
measured by a scale is Iths of an inch = 550 feet, while the
breadth of ground occupied by their outcrops measures
more than twice as much.^
Moreover, it is evident that we can also ascertain in the
same manner the depth of any given bed below any point
along this line of section. Suppose, for instance, that h o
is a bed of ironstone, and that we wished to determine the
depth at which it would be found in a perpendicular shaft
below the point d. By scaling off the length of the vertical
line under d, this depth is found to be about |ths of an
inch = 330 feet.
To make such a calculation it is not, of course, necessary
to construct a section on a definite scale. It is only neces¬
sary to observe the angle of dip and to measure the dis¬
tance of the given point from the outcrop of the particular
stratum, and the desired depth can be ascertained trigono-
metrically by the solution of a right-angled triangle of
those dimensions. To save trouble reference is generally
made to a depth and thickness table, which gives the
thickness measured at right angles to the dip, and the
depth measured at right angles to the horizon, for every
angle of inclination that any given bed will attain in a
' The term outcrop is often used for the tract of ground occupied
by the outcrops of a set of beds of the same litholoñcal character,
such as make up what is called a sandstone or a shale. Properly
speaking, however, the outcrop of the sandstone in fig. 108 is at
a, and the tract of ground between d and 2, which is formed by the
basset edges of the successive beds of sandstone, would be more
correctly termed the basset surface of the sandstone, as distin¬
guished from its natural surface, which is that covered by the
overlying shale.
CHAP. VIII.j INCLINATION AND FLEXUBE OF BEDS. 423
horizontal distance of 1,000 (feet, yards, etc.), measured
directly across the strike.
DEPTH AND THICKNESS TABLE.
Horizontal distance = 1000.
Angle of Dip.
Depth.
Thickness.
Angle of Dip.
Depth.
Thickness.
1
17
17
16
287
276
2
35
35
17
307
292
3
53
53
18
318
309
4
70
70
19
345
326
5
88
87
20
366
.342
6
106
105
25
469
423
7
123
122
30
580
500
8
141
139
35
705
574
9
160
157
40
842
656
10
177
174
45
1000
707
11
195
191
50
1190
766
12
214
208
55
1430
819
13
232
225
60
1740
866
14
252
242
65
2140
906
15
269
259
70
2750
940
Folds and Flexures.—Although beds sometimes con¬
tinue to dip at nearly the same angle for several miles, yet
it will generally be found that the whole series is bent into
a great curve, so that in crossing their lines of outcrop we
shall come to a point where the beds dip in an opposite di¬
rection. In low countries and broad table-lands these
curves are sometimes only gentle undulations ; elsewhere
more decided folds and flexures occur, while in mountain
regions the rocks are often bent or crumpled into a series
of wonderful convolutions or contortions.
When these folds are on a large scale they are spoken of
as anticlinals and synclinals ; an anticlinal fold being that
in which the beds are bent upwards into the shape of an
arch or saddle, and a synclinal fold where they are bent
downwards as into a trough. Such curves are always sup¬
posed to be bent upon an imaginary line which is called the
axis of the curve ; thus in flg. 110 the axis of a synclinal
424
STRUCTURAL GEOLOGY, [paet II.
curve is intersected at the point s, and the axis of an anti¬
clinal curve at the point a.
Fig. 109 is an ideal plan of a district to show the way in
which the outcrops of beds that are bent into anticlinals
and synclinals are repeated on either side of the axis ; it is
c s
a d
Fig. 109. Plan of Beds bent into an anticlinal and synclinal Curve.
part of a geological map in fact, markings or patterns serv¬
ing the purpose of colours, and fig. 110 is a section across it
along the line c d.
The beds are numbered from below upwards, and it is
evident from inspection that the beds or groups of beds.
Fig. 110. Section along the line c d in fig. 109.
and that the groups 2, 3, 4, are repeated on either side of
the anticlinal axis, a a. The arrows point in the direction
of the dip, and the actual dips are seen in the section, fig.
110, where the line c d is the surface of the ground,
chap. viii.J inclination and flexure of beds. 425
In figs. 109 and 110 the axes of the folds are supposed to
be parallel to the general strike of the beds, and conse¬
quently the folding only interferes with the uniformity of
the dip ; but it often happens that beds are folded into
curves the axes of which are nearly at right angles to the
2 34 5
Fig. 112. Section along the line c d in fig. 111.
a
Fig. 111. Plan of inclined Beds folded across the Strike.
general line of strike ; in such cases the axes are inclined,
not horizontal, and the folding interferes with the uni¬
formity both of strike and dip.
Fig. Ill is a plan or map of a series of beds which have
the same general strike as those in fig. 109, viz., from
E.N.E.
W.S.W.
5
W.S.W. to E.N.E., with a normal dip to the E.S.E., as in¬
dicated by the arrows. This strike and dip are, however,
interrupted by the anticlinal folds at a and b, which cause
the irregularities in the lines of outcrop by bringing up the
lower beds, 1, 2, and 3. The axes of these folds are them-
426
STRUCTURAL GEOLOGY.
[part ii.
selves inclined to the E.S.E. with the normal dip, so that
where the line of section crosses bed 2 three dips might be
observed, one to W.S.W., one to E.N.E., and one to E.S.E.
Fig. 112 is a section parallel to the general strike along
the line c d, showing the dips produced by the anticlinal
axes. A section anywhere at right angles to this would
of course show only the dip produced by the general in¬
clination of the strata to the E.S.E. Such cross rolls or
undulations are of frequent occurrence over the ground
occupied by the Jurassic and Cretaceous rocks in England.
Fig. 113. Plan of a periclinal Basin.
Periclinal Curves.—When the axis of an anticlinal
or synclinal fold is likewise curved, so that the beds are
bent up at each end as weU as at each side, oval-shaped
domes or basin-shaped depressions are produced which
have a qua-qua-versal dip, and may be called periclinals.^
Fig. 113 is a plan or map of a periclinal basin formed by
the outcrop of a series of beds which everywhere dip
inwards towards a centre, as shown by the direction of the
arrows. These outcrops run in concentric rings or bands
round the central area formed by the highest member of
the series. Fig. 114 is a section along the line a b, and
shows the manner in which the strata forming these con-
' This term was proposed by Dr. Page in his "Textbook of
Geology," and adopted by W. Topley in his " Geology of the
Weald, Geol. Survey Mem.," p. 216.
CHAP. TIII.J INCLINATION AND FLEXURE OF BEDS. 427
centric bands crop out to the surface ; a section taken
along any line across the periclinal would present a similar
appearance. A periclinal with inward dips is often spoken
of as a hasin ; but there is no recognized name for a peri-
cline with outward dips. Periclinals which are much
longer than they are wide may be spoken of as cymhoid
or cymhiform, that is, boat-shaped.
Monoclinal Curves.—When a set of horizontal beds
is suddenly bent up or down into a sharp curve, and then
Fig. 114.
continued horizontally at a higher or lower level, the flexure
is called a monoclinal or uniclinal curve. In the Isle of
Wight, for instance (see fig. 107), the strata which form
the hills in the southern part of the island are horizontal,
but in the middle of it they suddenly plunge down at a very
steep angle to the north, and then pass horizontally below
the newer beds which form the low ground in the north of
the island.
Even if the beds are bent back again by a second corre-
^iiii-Lmgg^
Fig. 115. Diagram of the Structure of the Kaibah Plateau,
Colorado.
sponding flexure, so as to resume their former level, the
form produced is not an arch, and though it might be
called a flat-topped anticlinal, yet it is better to consider it
as a combination of two monoclinals.
The region of the Colorado Plateaux in North America
affords many instances of single and double monoclinal
curves. The great features of the country, in fact, depend
upon them, for the strata are horizontal except where they
are bent upwards or downwards by such curves (see fig. 115).
The country is thus divided into great blocks, several miles
428
STRUCTURAL GEOLOGY.
[part ii.
in breadth, and many miles in length, which have been un¬
equally lifted, so that each differs from its neighbour in
relative altitude. These blocks are bounded either by mono-
clinal curves or by displacements of the strata ( = faults).
Pig. 115 is a representation of the Kaibab block, copied
from a woodcut in Major Powell's Eeport of his "Explo¬
ration of the Colorado." Its upper surface constitutes the
Kaibab Plateau, which has a width of about 15 miles, and
the strata of which it is formed are flexed down along both
margins. The plateaux which compose the Colorado pro¬
vince reach altitudes of 5,000 to 11,000 feet above the sea,
so that some of the blocks into which they are divided form
true mountain ranges, such as the Uinta mountains.
Isoclinal Curves and Inversion of Beds.—These
flexures are in some instances carried out so far, both on
the small and large scale, that the beds are more or less
inverted, and the lower surfaces of beds may be mistaken
for their upper surfaces. Thus, in fig. 116, beds of shale
AB c
and limestone are bent into folds which are tilted over in
one direction, so that the same bed is brought to the sur¬
face again and again, and an observer who walked over the
ground might be deceived into thinking that he was dealing
with a great thickness of beds all dipping uniformly in
one direction, until he came across some quarry or open
section which showed him the true structure of the
country. A structure like this, where the folds are all
inclined in one direction, has been termed isoclinal. At c
the folding has been so great as to produce complete inver¬
sion. Such inversion may sometimes be actually seen in
cliffs, as in North Devon, or in the sides of mountains, as
in fig. 117, which is a sketch of the curved strata seen
in certain Alpine peaks by Professor Bonney.^ In other
^ See his " Alpine Regions," p. 21.
CHAP. VIII.J INCLINATION AND FLEXUKE OF BEDS. 429
cases it requires a more widely extended observation in
order to show that the apparent order of superposition is
the inverse of that which is to be observed in other locali¬
ties where the beds are less disturbed.
The inversion of beds is occasionally observed in coal¬
mining, as in Belgium and the south-west of Ireland, where
beds of coal are sometimes found with the " coal-seat " or
underclay uppermost, and the coal-roof undermost. In such
cases, when a vertical shaft is sunk, it frequently happens
that the same bed of coal is passed through a second time
Fig. 118. Fig. 117.
Views of inverted Strata.
at a lower level, the beds below it being a repetition of
those that were first found above it. Thus, in a disturbed
part of the South Staffordshire coal-field, the same bed of
coal was passed through three times in the same vertical
shaft, first in its right position, then inverted, and thirdly,
right side uppermost : it must, accordingly, have been bent
into the shape of the letter S, as in fig. 118. It is stated
that in a coal-mine at Möns, in Belgium, the same seam of
coal has been perforated no less than six times, so narrow
are the plications into which the strata are folded.
Fig. 119 is a case observed by the writer in the cliffs
between Hfracombe and Eillage in North Devon. In the
face of the cliff fronting the sea there appeared to be three
430
structural geology.
[part ii.
bands of thin-bedded limestone (l) interstratified with black
shales (b) ; but the side view, seen in a little cove, showed
that there were only two bands of limestone in reality, and
that the beds had been rolled together and inverted as
represented in fig. 119, b.
Contortions.—Large anticlinal and synclinal cmwes
often have minor undulations and flexures on their flanks,
and when these are on a small scale, so that several
can be seen in the quarry or cliff section, they are termed
■contortions. These contortions are sometimes so close
together that the strata look like the crumpled leaves of a
book ; good examples are figured in Jukes' " School Manual
of Geology," 1890, p. 151, and Green's "Geology for
b a
Fig. 119. Inversion near Ilfracombe.
a. Face of cliff, b, Side view.
Students," 1876, p. 346. Several similar instances are
observable in the cliffs near Ilfracombe, and no better
coast than that of North Devon could be chosen for study¬
ing the effects of flexure, contortion, and inversion. The
coasts of Wales, Ireland, and Scotland offer equal facilities
for such a study.
Shales which are interstratified with thin beds of lime¬
stone or sandstone are sometimes crumpled and folded so
sharply as to form a succession of oblique Z's or Vandykes ;
when these are exposed in plan on the sea-shore they look
like a series of troughs or boats, each fitting inside the
other like a nest of boxes. In such cases the shales are
often squeezed out from between the harder beds so as to
form pockets, and Mr. Jukes mentions an instance in the
CHAP. VIII.J INCLINATION AND FLEXURE OF BEDS. 431
collieries near Kanturk, county Cork, where beds of culm
and anthracite, which were originally perhaps 2 or 8 feet
thick, expand in some places to a width of 20 or 30 feet,
while at others they dwindle down to a single inch.
Fan Structure.—In mountain chains the rocks have
sometimes been so folded and compressed that a double
series of isoclinal flexures has been developed ; the axes of
the folds in each series sloping inwards on both sides of a
great central anticlinal fold. This arrangement has been
termed "fan structure," and is well exemplified in the
Mont Blanc range, through which fig. 120 is a diagrammatic
section.
It will be seen that the newer beds (a a) actually dip
beneath the older rocks which form the central mass, and
therefore the relative age of the groups might in such
cases be entirely misunderstood by anyone unaccustomed
to interpret the structure of mountain ranges. ' In such
compressed and convoluted districts the folds are often
puckered by minor contortions and dislocations which add
to the complexity of the structure and greatly increase the
difficulty of discovering the correct succession of strata.
Cause of Flexures and Contortions.—During the
movements of upheaval and subsidence which are con¬
stantly taking place in the earth's crust, as described in
1. Brévent.
2. Valley of Chamouni.
Fig. 120. Fan Structure.
3 and 4. Massif of Mont Blanc,
louni. 6. Val de Vini.
^ See " The Secret of the Highlands," hy Professor C. Lapworth,
in "Geological Magazine," Dec. 2, vol. x. pp. 199, 337.
432
STRUCTURAL GEOLOGY.
[part ii.
Part I., CEapter IV., the rocks must necessarily be sub¬
jected to great strains and pressures.
Though the earth as a whole seems to be extremely
rigid, yet the rocks of which the crust is composed are by
no means rigid ; all clays and shales are more or less plastic,
and many sandstones can be bent to a certain degree with¬
out fracture. Further, in many cases it is probable that
the rigidity of the harder rocks has been caused by the
compression of their mass. Hence we may conclude that
the earth's crust is plastic within certain limits, and that
folding and crumpling of its materials is possible under
certain conditions.
It has been suggested that the mere vertical subsidence
of a large area of the earth's surface will be sufficient to
cause much flexure and contortion. Mr. J. M. Wilson has
considered the results of subsidence,' and he argues that
the borders of such an area must be subjected to lateral
pressure and that the rocks of which it consists must be
more or less folded and contorted. He therefore concludes
that " contortions are the inevitable result of the subsidence
of a curved surface."
If, however, we examine his results it does not appear
that mere subsidence is capable of producing much com¬
pression. He constructed a table to show the amount of
compression which would result from a given amount of
subsidence, and from this it appears that if a circular region
of the earth's crust measuring 690 miles across the surface
were depressed through one mile (5,280 feet), its mass
near the surface would be compressed into a space which
had a diameter narrower by 633 feet. It follows from
this that every bed of rock in this tract would be com¬
pressed to the amount of 1 foot in every 5,800 feet, and if
the region were originally 2,760 miles across the compres¬
sion would only be 1 foot in 6,000. This is so small an
amount that it could all be expended in consolidation with¬
out flexure. Even if we assume a depression through four
miles (21,000 feet) the amounts of compression are only I
in 1,510 and 1 in 1,550 respectively, and this would go a
very little way toward producing the amount of flexure
' " Geol. Mag.," vol. v. p. 206.
CHAP. VIII.J INCLINATION AND FLEXURE OF BEDS. 433
represented in the preceding figures ; for an ordinary
anticline like that represented in fig. 110 means a compres¬
sion of 1 in every 3 feet, that is to say, 3 feet are bent on a
chord of 2 feet, and 300 on a chord of 200 feet.
It is evidentthat for such results we require a much greater
compressive force than is supplied by the mere depression
of a curved surface ; a greater force may be found in the
secular contraction of the earth's crust, which follows from
the cooling of the earth as a whole, but Mr. Fisher has
shown that this could not do all the work that has been
done, and he suggests that local pressures are exerted by
convection currents in the liquid substratum upon the
under surface of the crust. The consideration of this
matter must be deferred to a later chapter ; at this stage
the student must be content with the fact that plication
can be produced by lateral pressure applied under the con¬
dition of g^'eat vertical weight, as was proved long ago by
the experiment of Sir James Hall. He took pieces of diffe¬
rently coloured cloths, placed them horizontally on a table,
and covered them by a weight (see fig. 121). He then
applied pressure at the sides (i> h, fig. 122), and found that
while the weight was slightly raised the cloths were folded
and contorted in a manner exactly analogous to the foldings
and contortions observed in nature and illustrated by the
preceding figures.
It must be remembered that the rocks which now appear
so intensely crumpled and contorted have been buried
under vast thicknesses of other rocks, and consist of various
F P
434
STRUCTURAL GEOLOGY.
[PAET II.
beds of unequal hardness, which would offer unequal
resistance to the force of lateral pressure. Moreover, the
disturbances which caused the pressure have been repeated
at many periods of geological history, so that the older
rocks of a region that has been subjected to many such
disturbances are naturally more bent and twisted than the
newer beds.
Thus in the south and east of Ireland there are three
great rock-groups, which have been named respectively
the Cambrian, Ordovician, and Carboniferous ; it can be
shown that the rocks of the first were greatly disturbed
and contorted before the deposition of the Ordovician, that
the latter were similarly plicated before the deposition of
the Carboniferous, and that the Carboniferous have also
themselves been bent and folded into numerous curves.
It is not, therefore, surprising that the Cambrian beds
should in many places have been twisted into a confusion
of knots and contortions, the complete unravelling of which
would be a perfectly hopeless task.
There is, however, one kind of flexure which does not
appear to be caused by direct lateral pressure, or rather it
may be due to the conversion of a lateral pressure into a
vertical uplift. This is monoclinal flexure, as illustrated
by fig. 115. To this we shall recur in the next chapter.
CHAPTEE IX.
FAULTS OE DISLOCATIONS.
I. Direct or Normal Faults.
IF great thickness of hard and solid rocks can be bent
into the curves and contortions described in the last
chapter, it may easily be conceived that a difEerent apphca-
tion of the same force would be capable of cracking and
breaking through them. We find accordingly that rocks
are often traversed by great cracks or fissures, which pro¬
duce not only a severance, but a displacement of the beds
on either side, so that the separated portions are raised
or depressed far above or below those with which they
were originally connected. These fractures and displace¬
ments of large masses of rock are called dislocations or
faults.
Throw and Hade of Faults.—Suppose fig. 123 to
be a vertical section through a mass of rock dislocated by
two faults, s s being the surface of the ground, and c c the
course of a single bed which was once continuous, but has
436
structural geology.
[part ii.
been broken in two places by the two faults / a and / 6.
The two sides of any fault are spoken of as the up-cast and
down-cast sides, though it is generally impossible to say
whether the dislocation has resulted from the depression
of one side or the upheaval of the other. In fig. 123, the
mass resting upon the base, h a, may be supposed to have
been bodily upraised.
To indicate the amount of displacement caused by a faults
miners speak of its throw, and always measure this by the
vertical distance between the broken ends of any given bed.
Where the plane of the fault is vertical, as / a, in fig. 123^
the amount of throw is easily measured along it ; but when
the plane of the fault is inclined to the horizon, as at / h,
the throw is measured by prolonging the level of the given
bed till a vertical line will reach its continuation either
above or below it. In the case figured the throw of each
fault is supposed to be the same, viz., 100 feet. The beds
are also drawn horizontal, and when this is the case the
amount of throw is also measured by the thickness of the
beds displaced by the fault, i.e., the thickness between h
and d.
The student must observe, however, that the thickness
of the beds displaced is not always a measure of the throw
of a fault ; when the beds are inclined, as in fig. 124, the
vertical throw of the fault, measured from the level of one
end of the broken bed to the level of its continuation,
i.e., from a to h, is evidently less than the the thickness of
the beds displaced.
Miners also in describing the "ividth of a fault" mean
the horizontal distance between the broken ends of a bed,
namely, the distance c d in fig. 123. Geologists require a
CHAP. IX.J
FAULTS OR DISLOCATIONS.
437
more logical nomenclature, and often want to express the
amount of stratigraphical displacement, as measured by
the thickness of beds displaced, Messrs. Margerie and
Heim'^ have therefore proposed to distinguish the three
different measures of displacement above mentioned by
"the terms vertical throw, horizontal throw, and stratigraphical
throw. Thus, in fig. 124 the vertical and horizontal throw
are nearly equal, but the stratigraphical throw is much
greater than either.
The amount of displacement or " throw " varies from a
few feet to many hundred feet ; faults involving a dis¬
placement of even several thousand feet having been
found in some places, so that rocks of widely different
kinds and ages are brought into apposition with one
another. Thus, there is every gradation between cracks
and fissures, which are merely enlarged joints with hardly
any vertical displacement, and dislocations on the great
scale above mentioned.
The inclination of the plane of the fault from the
vertical, i.e., the slope from / to h, in figs. 123 and 124, is
called its hade or underlie. Thus we speak of the dip of
a bed and the hade of a fault, but their angles are not
measured in the same way. Faults usually hade at a high
angle, only 20° or 30° from the vertical being common in¬
clinations ; and they are sometimes vertical, as at / a in
fig. 123.
Open and Close Faults.—When faults traverse soft
and yielding beds of rock, such as shales and thin sand¬
stones or limestones, the fissures themselves are often mere
planes of division no thicker than a knife-blade.
When faults traverse very hard and unyielding rocks,
such as thick limestones or hard flagstones, and still more
if they penetrate hard igneous or metamorphic rocks, the
fissures are apt to be much wider and often very irregular.
If the original fracture has taken place, not in one plane,
but so as to produce two uneven and irregular surfaces,
these surfaces, in sliding one over the other, are not likely
to fit very closely, but will leave hollows and spaces here
and there between the two walls of the fissure.
"Dislocations de l'éoorce terrestre," Zurich, 1888, p. 17.
438
STRUCTURAL GEOLOGY.
[part ii.
It is true that the grinding process, as the rock-surfaces
moved across each other, would often greatly diminish
this irregularity, and, in soft rocks, probably obliterate it ;
but in hard rocks it is usual to find more or less space
between the walls of the fault.
In all faults the contiguous surfaces are generally found
to be polished and striated by the enormous friction which
took place during the movement of one face across the other.
These appearances are known by the name of slickensides,
and it often happens that such slickensides occur on the
surfaces of all the joints and cracks for some distance on
either side of the fault, and are indicative of the jarring
nature of the movement.
It must not be supposed, however, that such fissures
are now open and empty. Sometimes they have remained
open for a long time, and have been gradually filled up
with deposits of crystalline mineral matter, in which case
they are converted into lodes or mineral veins ; these
wüi be described on a future page. In other cases the
spaces between the rock-walls are filled with a confused
mass of fragments broken off the contiguous rocks during
the movement of dislocation, mingled with sand and clay,
produced by the attrition of these fragments against one
another and the walls of the fault. This mass of frag¬
mentary material is termed fault-rock, and is often con¬
solidated into a kind of breccia.
Very large lumps and blocks of the broken beds are some¬
times included in this way between the walls of a fault and
serve to indicate the existence and direction of the fracture
when it is being traced along the surface of the ground.
Single and Branching Faults.—In cases where the
dislocation is affected by a single line of fault, it is clear
that the beds must have been bent either upwards or down¬
wards on one side of the fault, or upwards on one side and
downwards on the other for a certain distance. Thus in
fig. 125 some beds are supposed to have been cracked by
the fissure a h, and the part c to have been bent down,
but we might just as easily have supposed the part d bent
up, or both operations to have taken place simiiltaneously.
Without some such bending no dislocation could have
occurred.
CHAP. IX.J
FAULTS OR DISLOCATIONS.
439
The existence of such single-line faults is often proved in
coal-mining. Not unfrequently they split towards one or
both extremities as in the plan, fig. 126, where the main
fault is split into three branches at one end, and two at
the other. The figures represent the amount of the down¬
throw at each point in yards. Such faults generally have
one, but sometimes several points of maximum " throw "
A
\ 1
a h— h— / i
j-^ liil
1
c
P ■
Fig. 125. Diagram of Single Line of Displacement.
near the centre, and gradually diminish each way till they
die out. To produce more than one point of maximum
throw, we must suppose that there is an undulation of the
beds along one or both sides of the fault.
Single lines of fracture are probably much more ex¬
tensive than the spaces where they are made visible by
the dislocation of the beds, since the bending which causes
Fig. 126. Plan of Fault splitting at the Ends.
dislocation is more likely to occur near the central part of
a fracture than near its extremities. Thus in fig. 134 the
fracture indicated by the line g is probably continued
beyond the point where the line ends, though the actual
dislocation ceases there.
When there is more than one line of fracture the fact of
dislocation becomes more easy to understand, since there is
no difficulty in conceiving that the angle or comer of
ground included between the intersection of two faults, has
440
STRUCTURAL GEOLOGY.
[part ii.
been dropped down below, or squeezed up above tbe cor¬
responding beds on the outside of them. In the plan fig.
127, let a b, c d, be two faults meeting in the point d, the
included part, e, being depressed below the level of the
outside parts. Even in this case, however, the beds inside
the two faults must be bent down in the direction of e d,
because, as the two faults end or die out at a and c, the
whole of the beds must be on the same level there, and
gradually change that level in proceeding in the direction
e d. The figures, as before, indicate the amount of throw.
In order to have any mass of beds entirely cut off on all
sides from those that surround them, and wholly depressed
below or raised above them on every side, it is obviously
necessary that we should have at least three straight faults
or two curvilinear faults surrounding the fractured piece
of ground.
Compound Faults.—A long powerful fault is often
composed, in the whole or part of its course, of a number
of parallel fissures, very close together, along a narrow
band of country, breaking the rocks into a corresponding
number of steps, as in fig. 128, which either throw all in
the same direction, or else, having some steps in an oppo-
chap. ix.J
FAULTS OR DISLOCATIONS.
441
site direction, produce a balance of throw in one direction,
so that it is treated as one wide fault.
Sometimes a set of strata are repeated again and again
by a succession of parallel faults much farther apart than
those in fig. 128, but hading in the same direction, so as
to form a series of broad steps, as in fig. 129. Such step
faults might lead an inexperienced obserrer, who traversed
the slope from E. to W., to think that the outcrops of the
bed a were those of four distinct beds, one passing beneath
the other, whereas in reality they are portions of a single
bed broken up and carried down to different levels.
If two neighbouring faults hade in opposite directions,
the two planes must meet along some line below the sur¬
face, and will produce what is called a trough fault. In such
a fault the trough piece, as it may be termed, is always let
Fig. 129. Step Faults.
down like a wedge between the outer masses of rock, as in
fig. 130.
The opposite faults of a trough may be either tmequal
in throw, as in the trough a, or equal in amount of throw,
as in the trough b. In the former case the displacement
along the Une, a, c, affects the whole mass of the surround¬
ing rock, as may be seen by tracing the beds x and a
through the dislocations ; in the latter case the displace¬
ments counterbalance one another, and affect only the
trough-piece b, which is included between the faults.
Mr. Topley, however, has pointed out that the appear¬
ance of a trough fault may be caused by the displacement
of one fault by another.^ It not unfrequently happens
that a district is traversed hy two or more sets of faults
produced at different times, and if one of the first set be
intersected by one of another set, a complicated series of
^ " Geology of the "Weald," Mem. Geol. Survey, p. 238.
442
STRUCTUBAL GEOLOGY.
[paet ii.
displaceinents may arise. Thus, in fig. 131, the bed, w, e,
was first dislocated by the fault a, b, c, causing a simple
downthrow on the w side at h. Subsequently a second
line of fracture was formed, d, e, f, which hades in the
opposite direction, so as to cause a displacement in the
b a a
Fig. 130. Trough Faults.
plane of the first fault. In the figure this second fault is
drawn with a downthrow on the e side, and the result of
the two fractures is to produce a trough fault, for if the
downthrow of the second fault was on the w side, it would
be a reverse fault, which is of rarer occurrence.
Downward diminution of Throw.—Where mining
operations are carried to a great depth it has been observed
that the throw of faults duninishes downward ; that is to
say, where a fault of the ordinary kind traverses a thick
series of beds, the distance between the broken ends of a
bed near the surface is greater than the distance be¬
tween the ends of another bed one or two thousand feet
CHAP. IX.] FAULTS OR DISLOCATIONS.
443
deeper. Mr. Curwen Salmon, writing to Professor Jukea
in 1862, stated that this is a general rule in Cornish mines
and in the lead mines of Northern England, and he inferred
that the result is due to subsidence of the downthrow side„
causing compressive extension of deeper seated strata.
Flexure of Beds near Faults.—Small faults may
sometimes be observed where the beds have been broken
through so cleanly and evenly that they look as if they
had been cut with a knife, the opposing walls of the
fissure fitting closely and regularly to one another. Moro
generally, however, the contiguous strata have been bent,
crumpled, and pulled down along the plane of the fault
to a certain extent, as in fig. 132. The beds are here re¬
presented as bent upon the downthrow side, or in mining
language as " rising towards the upthrow," and " dipping
towards the downthrow," and this is naturally the
most usual effect of a forcible dislocation, accompanied
or followed by lateral pressure ; but it is not invariably
the case, instances occurring where the beds have been
bent in an opposite direction, so as to curve upwards on
the upthrow side.
Mr. Jukes has described a case of the latter kind as seen
in the gate-road of a colliery at Himley in South Stafford¬
shire.^ The beds here brought into contact by this fault
were the coal-measures and certain red rocks (known as
Permian). Mr. Jukes says, " Both formations were greatly
shattered and broken, so much so that no trustworthy de-
determination of the dip of the Permian beds could be
made. The coal-measures were not only shattered but
Fig. 132. Flexure of Beds near ä Fault.
' " Manual of Geology," third edition, p. 201.
444
STRUCTURAL GEOLOGY.
[PAET II.
squeezed, so that the thick coal lost much of its usual
thickness on approaching the fault, and when it came
within 10 or 12 yards of it, it was bent up perpendicularly,
and the beds below it rose into the walls of the gate-road,
and were cut off above by the red rock lying obliquely
across them at an irregular line, the inclination of which
to the horizon did not exceed 37", somewhat as in the pre¬
ceding figure (fig. 133), which is condensed from a rough
sketch and measurement I made on the spot."
Effects of Faults on Outcrop.—Since rocks rarely
remain horizontal over any considerable area, it is neces¬
sary to study the effects produced when a series of inclined
or curved beds are traversed by a fault. It it obvious that
the displacement must afl'ect their outcrops, but the results
produced will vary according to the following circum-
A BCD
Fig. 133. Section through a Fault at Himley (after Jukes).
A, The Thick coal squeezed and bent upwards ; B, Ironstones and
shales ; C, The Heathen coal ; D, Permian beds.
stances:—1, The direction of the hne of fracture; 2, the
inclination of the strata ; 3, the amount of the vertical dis¬
location. When the line of fracture cuts more or less directly
across the strike of the strata, it may be called a transverse
or dip-fault ; and when it is nearly parallel to the strike of
the beds, it is called a strike fault. Those which traverse
the strike diagonally may be called oblique faults, and will
produce a more complicated shift. Most faults, however,
may be treated either as transverse or strike faults, and it
will be sufficient, therefore, to describe the surface effects
produced by these two classes under varying conditions of
dip and flexure.
Transverse Faults.—If a set of beds, dipping at a given
angle, and striking in a given direction, be traversed by a
transverse fault, the effect of the vertical throw is to pro¬
duce at the surface the appearance of a lateral shift. Let
chap. ix.] FAULT.S OR DISLOCATIONS.
445
fig. 134 represent the plan of a small tract of country, on
which are drawn the outcrops of a certain set of beds,
sandstones, shales, and ironstones, having a north and
south strike, and an easterly dip as indicated by the arrows.
These beds are broken by two faults, indicated by the
lines p, g, the latter of which dies out in the area. The
effect of these dislocations is to shift the out-crops to the
eastward, but the actual movement was simply a vertical
one, both the faults having upthrows to the north, or
downthrows to the south, which is the same thing.
It is difdcult to make this quite clear to the reader with¬
out the assistance of a model, but it would not be difiBcult
for the reader to construct such a model for himself, re-
Fig. 134. Plan to show Shifting of Outcrop by Transverse Faults.
membering that after the dislocation is produced, the
upheaved piece must be cut through along a plane co¬
inciding with the horizontal surface of the other piece.
Suppose such a model to be represented in fig. 135, g n o p
being the plane of the fault, the block, a, being on the
downthrow side, and the block, b, on the upthrow side of
this fault. Let the plane, c d e p, be part of an inclined
stratum, the corresponding part of which, on the upthrow
side, is the plane, m n o p ; the line, m n, having been
the continuation of the line of outcrop, c d.
It is clear that if the upper portion of the block, b, is
cut off along the plane, g k i, so that the surface of both
blocks be again in one plane, then the outcrop of the
p G
446
STRUCTURAL GEOLOGY. [part ii.
iuclined stratum on the block, b, will coincide with the line,
K L, and will be in advance of the outcrop, c n, by the
horizontal distance, n k ; just as in fig. 134, the outcrop
of the bed, a a, is thrown eastward on the upthrow side of
each fault.
The effect of the fault may be grasped by the reader if
he will hold two flat pieces of wood or cardboard in an in¬
clined position and imagine that they represent the bed of
coal indicated by the thick black lines, fig. 135. If heathen
lift one vertically about an inch above the other, but still
keeping them both at the same angle, he will see that if
both were intersected by a horizontal plane, the outcrop
of one would be in advance of the outcrop of the other.
n
Fig. 135. Diagram to e.xplain Lateral Shift by a Vertical Throw.
He will also see that the higher the angle at which the
beds dip, the less wiU be the apparent shift at the surface
produced by the amount of throw. In fact the amount of
apparent lateral shift varies directly with the throw of the
fault and inversely with the dip of the beds, i.e., the dis¬
tance between the outcrops increases with the increase of
the throw, but decreases with the increase of the dip.
It is also apparent that if we know the distance between
the outcrops of the beds and their angle of inclination, it
will be easy to calculate the amount of the vertical throw,
and so to discover the depth or distance at which the one
chap. ix.j faults or dislocations.
447
part of the bed will be found above or below the corre¬
sponding part on the other side of the fault.^
That this apparent lateral shift is really due to vertical
displacement is more obvious when the fault traverses a
Fig. 136. Effect of a Transverse Fault on Double Outcrops.
district where some of the displaced beds are twice brought
to the surface, either by synclinal flexure, or by surface
inequalities, as on each side of a hiU.
Let flg. 136 be a ground plan representing the outcrop
of a bed, s s, round part of a long hill or ridge, and
w E
s
r
Fig. 137. Section along the line c D in fig. 136.
N.B. This should have been drawn as a normal fault hading to
the E.
traversed by the line of fault, p p. It is evident that if
the apparent shift of the bed, s s, on one side of the hill,
say at a, was due to an actual lateral movement, the out¬
crop of the bed on the other side (at 6) would have been
^ It is possible, however, that some dislocations may really be
lateral shifts without any vertical displacement. These may be
called heaves, and they appear to occur in districts which have
been much disturbed and fractured ; see " Reports of Miners
Assoc. of Cornwall and Devon, 1879, Phenomena of Heaves," by
A. T. Davies.
448
STRUCTURAL GEOLOGY.
[paet ii.
shifted in the same direction, i.e., farther north. No kind
of lateral movement could possibly bring the two outcrops
nearer one another, as is the case on the plan, but the sec¬
tion fig. 137, along the line c d on the plan, will show that
this effect is the necessary result of a vertical displacement
■with an upthrow on the western side. By such a displace¬
ment the whole of the block on the west side of the fault
is raised so as to bring the bed, s s, nearer the surface of
the ground, and, as a consequence, the outcrops of this bed
must be brought nearer together. If the fault had been a
downthrow on the west the distance between the outcrops
on that side would have been increased. In the same way
d
c
Fig. 138. Kepetition of Beds by a Strike Fault.
whenever synclinal and anticlinal curves are crossed by
transverse faults the distance between the outcrops in the
synclinals is lessened and in the anticlinals is increased on
the upcast side, while on the do-wncast side the reverse re¬
sult is produced. The greater the throw of the fault the
greater will be the difference in the distance between the
outcrops on either side.
Instances of these effects of transverse faults -will be
found by the dozen on the maps of the various coal-fields
and other disturbed districts published by the Geological
Survey of Great Britain and Ireland.
Strike Faults.—The faults hitherto figured and- de¬
scribed are such as cut across the beds so as to interfere
with the continuity of their strike ; but, as already stated.
chap. ix.] FAULTS OR DISLOCATIONS.
449
other lines of fault may run nearly, or quite parallel, to the
strike of the beds, so as only to interfere with the con¬
tinuity of their dip. These are called strike faults, and
their effect is generally to repeat the outcrops of the beds
which are so displaced.
This effect is illustrated by the plan and section, figs.
138 and 139, the plan representing a series of beds striking
nearly N.E. and S.W., and dipping to the south-east.
w.n.w. e.s.e.
Fig. 139.
Repetition of Beds by a Strike Fault along line C D in Fig. 138.
These are broken by the line of fault, f p, which is an
upthrow to the east, so that where the section is taken
along the line c d, all the beds which crop out on the
western side of the fault are repeated on the eastern side.
Such strike faults are frequently accompanied by a
change in the amount of dip, as in the figure, where the
dip on the western side is at an angle of 15°, while on the
eastern side it is diminished to 10°.
In the above instance the fault is readily detected in
consequence of the repetition of the beds, caused by the
fact that the fault hades against or under the dip of the
strata, and this is most usually the case. There are cases,
however, where the fault hades with the dip, i.e., in the
same direction, and then, if it also obeys the ordinary rule
of hading to the downthrow, an opposite result is pro¬
duced. Some of the beds are then concealed behind the
G Q
450 STRUCTURAL GEOLOGY. [PAET II.
fault, SO as to be prevented from cropping out to the sur¬
face at all. Thus in fig. 140 the outcrops of the beds
numbered 4 and 5 are wholly concealed from view ; and
without some natural cross section like that in the figure,
such a fault would be difficult of detection at the surface,
because the surveyor would be apt to think that the beds
1, 2, 3, 6, 7, were in continuous succession, unless he were
previously aware of the existence of the beds 4 and 5.
The same result would be produced by a reverse fault
hading in the opposite direction.
Magnitude and Extent of Faults.—Faults are some¬
times dislocations of very great magnitude, both as regards
amount of throw and the horizontal distance over which
they extend. As a good instance of a strike fault, we
GRS
Fig. 141. Section through Tipperary (after Jukes).
C M, Coal Measures. OES, Old Ked Sandstone.
C Ii, Carboniferous Limestone. S, Silurian.
select that known as the Slievenamuck fault in county
Tipperary, which is one of the largest in Ireland. This
great fracture extends for a distance of 25 miles, and has
a downthrow of nearly 4,000 feet.
Fig. 141 is a section across part of county Tipperary,
and represents the dislocation produced by this fault.
The rocks displaced by it are (1) certain slates and grits
known as Silurian rocks, (2) reddish sandstones, called the
Old Ked Sandstone, (3) grey limestones, called Carboni¬
ferous limestone, (4) black shales belonging to the Coal-
measures. On the south side of the fault the Old Ked
Sandstone is found rising steeply from beneath the lime¬
stone of the Vale of Aherlow and forming a hill called
Slievenamuck, 1,200 feet high. In this hill its beds are
CHAP. IX.J
FAULTS OR DISLOCATIONS.
451
■well seen, nearly from top to bottom, and tbeir total thick¬
ness cannot be less than 1,000 feet. On the northern slope
of the hill the bottom beds of the Old Eed Sandstone are
exposed, and may be observed to rest on the uplifted edges
of the Silurian slates, which are seen for some depth along
the face of the hill. But, on descending the hill a little
lower, we come suddenly on to the Coal-measure shales
dipping at a gentle angle to the south, and abutting
directly against the Silurian rocks. These shales are about
800 feet in thickness and rest evenly on limestones similar
to those in the Vale of Aherlow. Numerous beds of the
same Carboniferous limestone crop out to the north, form¬
ing the plains of Tipperary, and making up a total thick¬
ness of about 3,000 feet. Finally, from imderneath these
limestones, the upper beds of the Old Eed Sandstone again
emerge, forming a second, but much lower ridge of hills,
which may be called the Emly ridge. The amoimt of dis¬
placement, as measured by the united thickness of the
beds which are affected by the fault, is 1,000 + 800 -t-
3,000 feet = 4,800, and we may, therefore, estimate the
stratigraphical throw at about 4,000 feet.
In England a transverse fault known as the Tale and
Bala fault runs from the lowland of Cheshire, through the
Hundred of Tale in Denbighshire, through Bala lake and
Tal-y-llyn to the sea coast near Towyn in Merioneth, a
distance of nearly 66 miles. Its downthrow on the north¬
west side is estimated to be between 3,000 and 4,000 feet.
A strike fault of equal magnitude runs along the western
border of the Pennine Hills, from Brough in Westmore¬
land into Eskdale, for 130 miles, and is known as the
Pennine fault. Its maximum throw is nearly 6,000 feet.
In Scotland a powerful fault crosses the country com¬
pletely from sea and sea, and separates the more ancient
rocks of the Highlands from the newer beds which form
the Lowlands. Its length from Loch Lomond to Stone¬
haven is about 110 miles. Another great dislocation sepa¬
rates the Lowlands from the Southern Uplands, running
for nearly 100 mUes from Ayrshire into Peebles, and
having an estimated throw of at least 15,000 feet in one
part of its course.
Strike faults often form the boundary of one formation
462
STRtJCTURAL GEOLOGY.
[part ii,
Fig. 142. A Fault cut off by newer beds.
or group of beds for a long distance, and are then termed
" boundary faults."
Age and Growth of Faults.—It often happens that
one formation is traversed by a series of faults which do
not pass up into the next overlying formation. Thus the
beds numbered 1 to 4 in fig. 142 are broken by a fault
which does not affect the beds numbered 5 to 8. It is
clear that the lower and older strata were fractured and
faulted before the deposition of the newer series. If we
call the beds 1 to 4 Carboniferous, and the beds 6 to 8
Jurassic, it is evident that the fault is of post-Carboniferous
and pre-Jurassic age.
Sometimes, however, we find a fault passing up through
a superior set of strata, but with a diminished amormt of
throw, showing that the displacement was produced partly
before and partly after the formation of the newer rocks.
Hence a fault which traverses rocks belonging to an early
period, and has a throw of several thousand feet, may not
have been caused by one great disruption, but may be the
result of many movements happening at successive but
distant epochs of time, for it is obvious that when once a
line of fracture and faulting is established it will always
be a line of weakness, and subsequent movements are
likely to prolong it, and to increase its throw.
This being so, we might expect that faults of great
magnitude would be found most frequently in areas where
ancient rocks are exposed, and this is actually the case.
CHAP. IX.J
FAULTS OR DISLOCATIONS.
453
2. Reversed Faults and Thrust Planes.
The faults -which we have hitherto considered all belong
to one class : they all have the same relation between their
hade and the direction of throw, the hade being invariably
towards and under the downthrow. When strata are
faulted in this way the ends of the severed beds are sepa¬
rated by a greater or less horizontal distance, called the
horizontal throw or " width of the fault," and no part of a
severed bed is ever brought vertically underneath another
part of it.
In some districts, however, another class of faults is
found which differ from normal faults in many important
particulars. They hade over the downcast side and under
the upcast, the upthro-wn portion of the beds being carried
along and up the plane of the fault, so as to lie above the
other portion, see fig. 143. This is called a reversed fault,
and by such faulting rocks are made to occupy less hori¬
zontal space, instead of being spread out over more space,
as in the case of normal faulting ; this will be apparent
from a glance at fig. 144, which represents a series of reversed
faults. It is evident that instead of any horizontal space
existing between the broken ends of a bed, one of these
ends actually overlaps the other. Again, the vertical throw
cannot always be measured by the vertical distance between
the two ends of a severed bed, but might be measured by
a vertical hue drawn from the lowest extremity of a bed
to the base of its overlying continuation. To the geologist,
however, who is engaged in surveying a district so faulted,
it is of prime importance to ascertain the amount of thrust.
Fig. 143. A Reversed Fault,
454
SÏEUCTimAL GEOLOGY.
[paet ii.
measured along the plane of the fault, between the two
ends of any severed plane of bedding.
Such faults, having been produced by a lateral thrust
which has forced the severed mass over its original con¬
tinuation, are sometimes called "thrust faults," but it
would be best to restrict the term thrust fault or thrust
jflane to a dislocation which cuts off and truncates several
reversed faults, as in fig. 144, where t t indicates a thrust
plane, and e e a series of truncated reversed faults.
This kind of faulting occurs on a wonderful scale in the
north-west of Scotland, the detailed structure of which
has recently been made known by the work of the Geological
Survey in that region.^ In some places several successive
series of reversed faults are found, each set being truncated
Fig. 144.
by a thrust plane, as in fig. 144. By the surveyors these
displacements are classified as (1) minor thrusts or reversed
faults, (2) major thrusts, (3) maximum thrusts ; but it is
less confusing to speak of them as (1) reversed faults, (2)
minor thrusts, (3) major thrusts. The reversed faults
repeat the strata by bringing lower to rest on higher beds,
and they lie obliquely to the thrust planes. The thrust
planes have a lower hade than the faults beneath them,
so that all the underlying strata with their system of
reversed faults are cut off by the thrust plane, and the
rocks above that plane have been carried bodily along in
one direction. In Scotland the strata have always been
carried from east to west. Lastly, there are major thrust
planes which actually truncate the minor thrust planes, so
^ "Quart. Journ. Geol. See.," 1888, p. 378.
CHAP. IX.J
FAULTS OR DISLOCATIONS.
455
that the country consists of a number of long wedge-shaped
masses of faulted strata, some of which have been carried
for many miles over the others, and the original order of
superposition is thereby completely inverted.^
Origin of Faults.
In endeavouring to explain the manner in which faults
have been produced, it is necessary to consider each of the
two great classes of faults separately, for it is obvious at
the outset that displacements which differ in so many
points are not likely to have been produced in the same
way.
1. Normal faults may be regarded as adjustments of
the strata to conditions which require them to spread out
over a wider area than they originally covered. Now there
Fig. 145. Diagram of a Region faulted by upheaval.
is only one natural operation which tends to produce such
conditions,namely,upheaval. When a portion of the earth's
surface is upheaved, however that is accomplished, the
upper layers of the crust are bulged out and compelled to
stretch out over a larger surface. As the rocks composing
these layers have very little elasticity, the tensions set up
during upheaval must soon result in numerous fractures,
and as the tension is greatest at the surface and less lower
down, there will be a greater amount of fracturing in the
upper than in the lower layers ; moreover, as the severed
blocks have to occupy a wider space, the fractures produced
will be such as to enable the masses to settle down till
they once more support each other (see fig. 145).
^ Fig. 176 is a generalized section across the north of Scotland
showing the long slices of faulted rock which have heen thrust one
over another from east to west. The hase of the slice mm is s.
major thrust plane which cuts off three minor thrust planes helow it.
456
STRUCTURAL GEOLOGY.
[PAET II.
During elevation also another influence comes into play
which tends to reduce the mass or bulk of the rocks com¬
posing the raised area, and thereby relatively to increase
the space over which they have to spread ; this is contrac¬
tion consequent on drainage and desiccation (see p. 390).
The rocks are at the same time rendered more brittle and
more liable to crack, and the fractures developed would
become faults as the blocks settle down against one another.
We may therefore conclude that the simple elevation of
a portion of the earth's crust would generate fractures
which would be converted into dislocations of the kind
known as normal faults.^
A case may be mentioned where simple upheaval seems
to have produced a series of faults of the normal kind,
such as would result from the settling down of a curved
tract. The island of Barbados in the West Indies consists
entirely of Tertiary strata. They fall naturally into three
groups or series ; the oldest are rocks which were formed in
shallow water, the next series consists of consolidated oozes
which were formed in oceanic depths, and the newest are
coral limestones which envelope the surface formed by the
two older groups, this surface having the shape of a flat¬
tened or low-arched dome. It is clear that after the for¬
mation of the oldest group of rocks the area subsided to a
great depth (probably about two miles) ; after remaining
for a long time as part of the ocean-floor, it was finally
raised into the arched surface part of which forms the exist¬
ing island of Barbados. Thus the history of the island
invests its faults and flexures with a peculiar interest : now
the oldest rocks are greatly flexured and contorted; the
oceanic beds are only tilted or gently flexured, but they are
cut up by faults and separated into blocks which are wedged
in between masses of the older series. These faults do not
affect the coral limestones, and they must, therefore, have
been formed during the upheaval of the island to the sea-
^ Mr. Mellard Reade has pointed out that wherever rocks have
heen expanded by heat, whether caused by a local rise of the iso-
geotherms or by the heat of rising columns of lava, when this heat
decreases the rocks will cool and contract ; faulting will thereby
ensue, and the faulted blocks will subside on the side farthest from
the source of heat.
chap. ix.] faults ob dislocations.
457
level. There is no evidence of any lateral pressure or of
any other kind of movement but upheaval since the forma¬
tion of the oceanic deposits.
Let us now fix our attention on a single line of fault.
Suppose that in the diagram, fig. 146, we have a section
of part of the earth's crust of which a b is the surface and
upheaval of the part a b c d. If, then, a fracture take
place along the line e f, it is obvious that the mass i will
present the broader base c f for the upheaving force to
act upon, and also that this mass grows smaller towards
the surface ; while the mass k has the smaller base and
the wider surface. It is clear, therefore, that it is easier to
lift the mass i into the position a c e /, than the mass k
into the position g h b i>. The mass i, therefore, will
become the upthrow side of the fault, while the mass k
will occupy a lower level,, and will form the downthrow
side. Hence the rule given on p. 453, that a fault
generally hades on the downthrow side.
^ This figure is copied from that in Jukes' " Manual of Geology,''
and the explanation is abridged from that given by the same
author.
458
STRUCTURAL GEOLOGY.
[part ii.
This is yet more clearly perceptible if we suppose two
such fissures, as in fig. 147, inclining towards each other,
since, if we conceive the included piece, o, to be elevated
into the position indicated by the dotted lines, it becomes
wholly unsupported, imless we suppose huge injections or
dykes of igneous rock to issue out along each fault, which
would remove the case from the class of fractures we are
at present considering. In the case represented by the
fig. 147, the masses a and b would both be elevated, and
the space between them would be widened, so that the
mass c would be let down along the planes of the two
faults, both of which would thus hade to the downthrow.
In this case, also, we have the key to the explanation of
certain trough faults. The reader must recollect that the
figs. 146 and 147 are merely diagrams to assist his com¬
prehension, and not actual representations, which would
necessarily exhibit a much greater amount of complexity.
The general conclusion that normal faults are produced
by the slipping and adjustment of fractured blocks during
upheaval is confirmed by the fact that faults have been
actually formed with the accompaniments of upheaval
and earthquake shock (see p. 60), the earthquake bemg,
apparently, the jar from the sudden yielding along the
fault plane.'
2. Reversed and thrust faults, as already stated, must be
regarded as adjustments which enable the strata to occupy
less horizontal space than they did originally, and it is also
clear from their mode of occurrence in Scotland that this
has been effected by the piling up of the fractured strips
of rock one over another, as in the diagram, fig. 148.
In this figure a b represents the position of a single
stratum before the faulting took place, and a c the position
of the severed fragments after the faulting, each broken
piece having been pushed up the plane of the fracture in
front of it. To get the arrangement shown in figure we
have only, then, to imagine the shearing off of the sub¬
jacent and superjacent continuations of the series by the
two thrust planes t, t.
' See C. Davison on British Earthquakes, "Geol. Mag.," 1891,
pp. 67 and 306.
CHAP. IX.J FAULTS OR DISLOCATIONS.
45»
The only force which could accomplish such an adjust¬
ment is a lateral thrust. It must, in fact, be the extreme
development of the lateral pressure which produces isoclinal
flexure, and the frequent association of isoclinal flexures
and reversed faults makes it clear they are due to the same
kind of impulse. The manner in which this force is sup¬
posed to be developed will be discussed in Part III.
Connection between Faults and Folds.—It has
been said that in a district which is much broken by faults
the rock-masses between them are not usually much con¬
torted. This may be true to a certain extent, but must
not be interpreted to mean that folds and faults are not
usually associated. On the contrary, there is a close rela-
Fig. 148.
tion between them; and the two great classes of faults
(striJce and transverse) may be regarded as connected respec¬
tively with the two classes of flexures described in the last
chapter. In the midland counties of England, where the
beds are bent into two sets of undulations, one parallel to,
and the other transverse to the general strike, there are
also two sets of faults, similarly related to one another.
In the Wealden area of Kent and Sussex the majority of
the faults have the same strike as the principal folds, viz.,.
from east to west.'
It is possible that many faults are only surface pheno¬
mena, and that, if traced downwards into the earth, they
would be found to pass into antichnal or synclinal flexures.
It is more hkely, however, that the relation between them
' Topley, " Geology of the Weald," Mem. Geol. Survey, p. 237.
460
STRUCTURAL GEOLOGY,
[part ii,
is in most cases an indirect one, that planes of weakness
running more or less parallel to the axes of the folds are
produced in the process of flexuring, and that these are
developed into fractures and faults by subsequent move¬
ments.
Between faults and monoclinal flexures there seems,
however, to be a closer connection, for these often pass
laterally into one another ; that is to say, the same line of
displacement may be a flexure along part of its course and
a fault along another part, the flexure gradually becoming
a fracture by the severance of the upper from the lower
limb of the curve. It is possible that monoclinal flexure
results in many cases from the displacement of a deep-
seated subterranean plane, which separates two very dif¬
ferent series of rocks. Suppose a b c in fig. 149 to be por¬
tions of a mass of ancient rocks which were indurated,
flexured, and faulted before the deposition of the newer
series d e, the line p p having been a horizontal surface of
erosion and the series n e being disposed originally in
horizontal strata above this plane. In the flgure it is
supposed that, under lateral pressure, the lower mass has
given way along the pre-existing faults, F p, and that the
block which lay between them has been forced to move
upward, so as to cause the vertical uplift of the superjacent
beds. It is evident that this tract will be bounded by
faults or flexures ; whether the one or the other is pro¬
duced "will depend partly on the thickness and pliability of
the strata forming the cover, and partly on the amount of
local uplift ; and it is quite conceivable that the displace¬
ment might be partly by faulting and partly by flexure.
P
e
f
Fig. 149. Faults and monoclinal folds.
f
CHAP. IX. j FAULTS OR DISLOCATIONS.
461
Mineral Veins.
In studying, faults our object has been chiefly to describe
their effect in dislocating the beds which they traverse, and
only brief mention was made of the minerals which they
occasionally contain. It was stated, however, that where
faults traversed hard rocks, they are generally more or less
open, the walls having been kept apart in some places by
their own inequahties. The same is the case with many
large cracks and joints in limestone rocks which have been
widened by the action of water.
Now, in many districts, these open flssures have become
the repositories of minerals that have been subsequently
introduced into them, and they are then termed lodes or
mineral veins.
The minerals they contain are usually in a crystalline
form, and consist of quartz, calcite, barytes, or fluor-spar,
together with the ores of one or more metals, such as
lead, copper, zinc, antimony, or silver. It is to the metallic
minerals that the miner, of course, chiefly looks ; and he
generally speaks of the earthy minerals as the gangue or
vein-stuff. The mineral contents of a vein are sometimes
confusedly dispersed through it, the vein-stuff occupying
the chief part of the space, and the ore occurring either as
disseminated crystals, or in nests or strings. Sometimes
there appears a regular arrangement of the various sub¬
stances : the cheeks or walls of the lode being lined with a
layer of crystals of one kind of substance, with their points
or apices turned inwards, each of these layers being covered
by a crystalline layer of another substance, evidently depo¬
sited upon the first ; and, after two or three such alterna¬
tions, a rib of ore is found in the centre (see fig. 150).
This kind of structure seems to involve the idea of suc¬
cessive depositions of the different coatings or linings of
the vein, the central rib of ore being the last or newest. In
some cases, it appears that fresh cracks were made by sub¬
sequent movements, and that new deposits were found in
these openings, slickenside surfaces often occurring on
their sides.
The term " lode " is the name applied to such mineral
veins in the West of England ; but in the northern parts
462
STRUCTURAL GEOLOGY.
[PAET II.
of England they are called raice veins, as distinguished from
pipe veins and flat veins, which are only irregular cavernous
spaces in the rocks that have been filled up with mineral
substances.
Lodes (or rake veins) vary gi-eatly in width and regularity
—sometimes they continue with an average width of 3 or 4
feet for a great distance, while other veins are subject to
twitches or closing-in of the walls, which alternate with
bellies or wide expansions of the vein. When such bellies
and twitches alternate very rapidly, they are spoken of as
waving veins.
When mineral veins traverse a succession of shales,
limestones, sandstones, etc., they are always found to be
narrower in the shales and sandstones, and widest in the
limestones, being sometimes so narrow in the shales as
to be only 6 inches wide, while in limestone, just above
■or below, they may be 2 or 3 feet wide. This, as Jukes
observes, seems obviously the result of the dissolving
power of water acting on calcareous, and not on siliceous
rocks, aided, perhaps, by the crushing-in of the softer
intervening shales.
It commonly happens that in a district containing mineral
veins two sets of fissures can be detected, those of each set
being parallel to each other, and crossing one another at a
constant angle ; the one set are called right veins, and the
other set cross veins or cross courses. The two sets of fis¬
sures may either be of contemporaneous origin, or one may
be subsequently formed.
Where right lodes and cross courses occur together in
a district, their contents, are often different, one kind of
CHAP. IX. J
FAULTS OR DISLOCATIONS.
463
ore being found in the former and another in the latter.
Where the date of the cross course is newer than that of
the lode, which is often the case, it is easy to understand
this difference. When, however, the two veins are con¬
temporaneous, as sometimes happens, it is not so easily
understood, but may perhaps be due to differences of tem¬
perature in the stream of water by which the minerals
were introduced into the fissure.
If a mineral vein be inclined from the perpendicular (and
they are seldom quite vertical) it is obvious that any sub¬
sequent fracture, producing dislocation, will displace the
first-formed lode just as if it were a bed. In the same
way its outcrops at the surface will appear to be shifted
laterally. Thus in fig. 151, c c is a right lode, the outcrop
of which is shifted by the fissure or cross vein, v v. With
regard to such intersections, Mr. Jukes observes that " if
Fig. 151.
the contents of the right vein are distinctly broken through
by the cross course, it is certainly strong evidence that the
cross course is newer than the right running vein. If the
contents of the cross course be continuous across the in¬
terrupted contents of the right running vein, the evidence
becomes still stronger;" but it appears that such is not in¬
variably, though very frequently, the case.
It should here be remarked that the hade or inclination
of lodes is always calculated from the vertical, as with
faults, and not from a horizontal line, as with beds ; a slight
hade, therefore, answers to a high dip, the one being the
complement of the other, a hade of 10° being a dip of 80°.
By what processes the various minerals have been intro¬
duced into veins is a subject on which much has been
written, but little is certainly known.
Sir H. de la Beche has pointed out that where warm
464
STRUCTURAL GEOLOGY.
[part ii.
water is rising from great depths through narrow fissures,
it must lose a large amount of heat as it approaches the
surface. Consequently those substances which are only
soluble at high temperatures, such as silica and many
metals, would crystallize out of solution, and would be
deposited on the walls of the fissure.
This theory finds support in the fact that mineral veins
are being actually formed in certain volcanic districts at
the present time. Sulphur Bank, California, is one locality
where such vein-formation has been observed.^ Hot springs,
solfataras, and fumaroles are here abundant, and the
deposits are clearly due to the uprising alkaline or solfataric
waters. At a certain depth below the surface, beyond the
influence of atmospheric agencies, the cracks and fissures
in the rock are filled with a hydrous silica (opal) in a soft
cheesy condition, generally streaked and clouded with
cinnabar (sulphide of mercui-y). In other parts of the
mine the silica has consolidated into chalcedony, and the
ciimahar is found sometimes in layers alternating with the
silica, sometimes lining or filling fissures by itself. Here,
therefore, veins with quartz vein-stuff and metallic ore are
now actually in process of formation.
It has been found that some relation exists between the
mineral contents of a vein and the nature of the rock which
it traverses. Some Cornish lodes contain copper ore where
the walls consist of slate, and tin ore only where they are
of granite." This fact may, however, be explained by sup¬
posing that different rocks are capable of determining the
deposition of different minerals on their surfaces under
the influence of terrestrial electricity.
' See paper by Leeonte and Rising in "Amer. Joum. of Science,"
Ser. 3, vol. xxiv. p. 23.
' J. W. Henwood, "Trans. Geol. Soc. Com.," vol. v. p. 190.
CHAPTEE X.
IGNEOUS EOCKS EEGAEDED AS EOCK-MASSES.
HE different kinds of Igneous rocks, regarded from
a lithological point of view, as hand-specimens in
which different minerals can be seen either without or with
a microscope, were described in Chapter III. In this and
the next chapter we shall treat them as rock-masses and
describe the positions which they occupy in the crust of
the earth.
A very little practical experience of igneous rocks in the
field teaches us that the most important point in which
such rock-masses differ among themselves is the relation
in which they stand to the stratified rocks by which they
are surrounded. In relation to the latter igneous rocks are
either intrusive or interbedded; that is, they have either
been injected or intruded among previously existing strata,
or else they are interbedded with strata which were formed
around the active volcano.
There is another point of difference which is important,
though not always so easily ascertained. Most igneous
rocks have been in direct connection with volcanoes, and
may he called eruptive, hut the subterranean reservoirs
from which they issued form, when cooled and crystallized,
deep-seated plutonic or hypogenous masses. It is often difiB.-
cult to say whether an isolated mass of granitoid rock was
or was not connected with an active eruptive vent, but there
is good reason to believe that many masses of crystalline
igneous rock are the terminal portions of intrusive magmas
which never reached the surface, but consolidated under a
thick cover of stratified rock.
H H
4(36
STRUCTURAX GEOLOGY.
[part ii.
In viewing the igneous roots as constituent parts of
the earth's crust, it seems most natural and convenient to
proceed from the known to the less known. Commencing
with the description of the remains of actual volcanic ori¬
fices which can be interpreted by our knowledge of recent
volcanoes, we may proceed to consider the more or less
circular intrusive masses which were once connected with
such vents ; thirdly, their prolongations in the form of
dykes, sills, and laccolites; and lastly, the interbedded
lavas which occur in acient volcanic districts.
a. Remains of Ancient Volcanoes.
Extinct Volcanoes.—It was stated on p. 136 that
modern volcanic cones, in common with all other terrestrial
surfaces, are wasted by rain and frost, so that their flanks
are trenched by deep gullies and ravines. As long as the
volcano continues to be active, these losses are more than
counterbalanced by fresh ejections, but when volcanic
activity finally ceases it is evident that such losses cannot
any longer be made good ; the mountainous pile of ashes
and lava-fiows will slowly succumb to the continued action
of the detritive agencies until nothing but a wom-down
stump is left to mark the existence of the former volcanic
cone. So long, however, as the district is not submerged
beneath the sea-waves, some remnants of the old volcano
will generally survive, and the existence of such extinct
volcanoes in various stages of ruin and decay has already
been mentioned.
Some still preserve their cones and craters, and the
lava-streams proceeding from them are still rough and
bristling, though they have never been active during
historic times ; others have had their cones and craters
more or less obliterated by the action of rain and frost, and
the country around them has been so lowered by general
detrition, and so altered by the erosion of valleys, that the
lava-streams which originally ran down the lowest groimd
they could find, are now found in detached remnants on
the top of ridges and plateaux.
No better example of such extinct volcanoes could be
CHAP. X.] IGNEOUS ROCKS AS ROCK-MASSES.
467
cited than those of Auvergne, in central France, sonae of
which are shown in the plate at the end of this volume.^
The Limagne d'Auvergne may be described as an exten¬
sive plain, chequered with low hills of fresh-water marl
and limestone which are capped with portions of lava-flows
(Plate II., fig. III.). This tract is enclosed on the east and
west by two parallel ranges of granite and gneiss, that to
the westward forming a lofty plateau of granite, rising to
a height of about 3,000 feet above the sea, and 1,600 above
the valley of Clermont. This granite plateau supports a
chain of volcanic cones and dome-shaped hills, about
seventy in number, varying in altitude from 600 to 1,000
feet above the plateau and forming an irregular range
nearly 20 miles in length and about 2 in breadth. The
highest point of this range is the Puy de Dome, which
rises to a height of 4,000 feet. Many of the cones retain
the form of well-defined craters, and their lava-currents
are as perfect as those of Vesuvius. Plate II., fig. II.,
is a view of part of the southern chain of Puys, as these
hills are locally termed ; several of them are broken down
on one side and lava-currents hate issued from the gap.
One of the most remarkable cones is the Puy de Come,
which rises from the plain to the height of 9Ó0 feet ; its
sides are covered with trees, and its summit presents two
craters, one of which is 250 feet in depth. A stream of
lava has issued from its base, but at a short distance,
having been obstructed by a mass of granite, it has
branched and flowed round it; thence it can be traced
along the granitic platform and down the side of a hill
into an adjacent valley, where it has dispossessed a river
of its bed, and constrained it to work out a fresh channel
between the lava and the granite of the opposite bank.
Mont Dore (Plate II., fig. I.) is a mountainous tract, the
highest portion of which is about 6,000 feet in altitude.
The dotted outline in fig. I. shows the presumed form of
the crater when in activity, but the crater is ruined and
broken down, and consists of a group of seven or eight
' Tlie following description of the volcanoes of Auvergne is
abridged from that in Mantell's " Wonders of Geology, and
founded upon Mr. P. Scrope's work on the " Geology of Central
France."
468
STRUCTURAL GEOLOGY. [PAET 11.
rocky summits, which form a zone about a mile in dia¬
meter. The mountain is deeply channelled by two princi¬
pal valleys, and is furrowed by many minor water-courses,
all originating near the central eminence and diverging
toward every point of the horizon. The whole mass con¬
sists of beds of scoria, tuff, trachyte, and basalt, all dipping
from the central axis and lying parallel to the sloping
flanks of the mountain, as in modem volcanoes.
Ancient Vents or Necks.—When a volcano has been
worn down by atmospheric and marine erosion, till only
the central neck or funnel, with perhaps some of the basal
tuffs and agglomerates, of the old volcano remained, and
the whole has been buried deep beneath subsequent accu¬
mulations, it will be entirely concealed from view within
Fig. 152. Diagram to show the manner in which wom-down
Volcanoes may be concealed and exposed.
the earth's crust, unless the whole tract has been again
upheaved and exposed by still later denudation.
It must, of course, participate in all the movements and
disturbances to which that portion of the earth's crust
may be subjected ; it may be tilted so that the inclination
of the stratified tuffs is altogether altered, it may be
broken and displaced by faults, and its lavas may be more
or less metamorphosed. When the region is again up¬
lifted, eroded, and denuded, the old volcanic vents may be
exposed as necks descending more or less vertically down-,
wards, as indicated in fig. 152 ; here a are the old rocks,
which were rent by volcanic action, and pierced by the two
colums of lava, c and d ; these columns once terminated
in volcanoes, of which only the basal portions now remain ;
6 are the newer rocks with which some of the volcanic
/
d
c
CHAP. X.J IGNEOUS KOCKS AS ROCK-MASSES. 469
products were interstratified, and e is part of an inter-
bedded lava-stream, which originally proceeded from the
vent that surrounded the neck, /.
The actual site of an old volcanic vent is sometimes
manifested, even after the total destruction of the cone
itself, by the exposure of the cyhndrical pipe which once
terminated upwards in the central orifice or crater of the
volcano. This pipe or chimney, when disclosed by denu¬
dation, is called a neck, and is always filled with some kind
of volcanic rock, either by a plug of solidified lava, or by a
mass of volcanic agglomerate. Such necks vary in diameter
from a few yards to several hundred feet.
A vertical section of a neck filled with crystalline rock
might be mistaken for a dyke, but a horizontal plan or
surface exposure would show that it is a round or oval pipe
and not a long fissure. In the case of necks filled with
agglomerate the nature of the material would prevent such
a mistake being made. In most cases the surrounding
rocks are partially baked or altered like those in contact
with all other intrusive masses, but there is one class of
necks in the neighbourhood of which no such alteration
has taken place.
There are several districts in the British Isles where
numerous volcanic necks exist, and where they have been
so dissected and laid open by coast erosion that their
relations to the surrounding rocks can be examined in
detail. County Antrim, in Lreland, and the shores of the
Firth of Forth in Scotland, are two of these localities.
Fig. 153 is a plan of a small neck of porphyrite in
Haddingtonshire, as exposed on the shore, near the mouth
of the river Tyne.' Its longest diameter is about 300 yards,
and a section through it would probably have the appear¬
ance shown in fig. 154. The beds of sandstone through
which it is drilled are baked and hardened, and it will be
noticed that they are bent down so as to dip everywhere
towards the central plug of lava. Near the neck the dip
is often between 40° and 50°, but at a httle distance it is
only 15° or 20.° This inward dip of the strata surrounding
a neck is of frequent occurrence, and m^y be attributed
^ " Grcology of East Lothian," p. 40.
470 STRUCTURAL GEOLOGY. [PAET II.
partly to the downward drag of the column of lava
caused by its contraction during the process of solidi¬
fication ; the weight of the ejected materials above may
also have helped to depress the subjacent strata.
Between the towns of Elie and St. Monaco, on the coast
Fig. 153. Plan of a Volcanic Neck.
A B, Low-water mark. C D, High-water mark. N, Neck.
S S, Sandstones.
The arrows indicate the dip of the sandstones.
of Fife, a magnificent exhibition of volcanic necks and
dykes is to be found between high- and low-water mark.
Close to St. Monaco there is a large boss of volcanic
agglomerate, with a plug of basalt in the centre, from
which numerous dykes of green-coloured basalt proceed in
various directions. Westward of this, and also exposed in
plan, is a small neck filled with basalt, where the mutual
effects of igneous and sedimentary rocks in contact are
beautifully shown, the shales outside being disturbed
CHAP. X.J IGNEOUS ROCKS AS ROCK-MASSES.
471
and porceUanized, while the basalt is bordered by a band
of white trap from 2 to 3 feet thick. Still farther west¬
ward are several other examples of small necks filled with
agglomerate, and projecting through the shales and sand¬
stones which form the shore, so that the observer standing
in the centre of one of them can easily take in the whole
of its circumference. These must have been mere pocket-
editions of volcanoes, and were probably only blowholes of
dust and stones, for the shales in contact with them
are not altered like those around the basalt necks.
The alteration of the stratified rocks in contact with
necks now filled with agglomerate, must have been effected
by the previous passage of molten lava through the pipe,
for it is evident that the passage of hot gases, dust, and
stones could hardly produce much effect on its walls. It
f
Fig. 155. Section across the Volcanic Basin of Ayrshire.
a, Older rocks. h. Sheets of porphyrite. c, Beds of tuff.
d. Bed sandstones, e. Volcanic necks. /, Dyke of basalt.
can sometimes be demonstrated that this was the case, as
in the following instance occurring in Ayrshire, and de¬
scribed by Professor J. Greikie.' " Outside the volcanic
ring (of interbedded ashes and lavas) there occurs a
number of small rounded hills or hillocks consisting of a
coarse red volcanic agglomerate. These hills are true
volcanic ' necks,' each representing a former crater or focus
of eruption. . . . They descend vertically through the
Coal-measures like so many huge pipes, sometimes standing
on lines of fault, while the coal near them is altered. In one
case, that of Helenton Hill, near Monkton, I found the
sides of the neck still partially crusted with a mass of
rough scoriaceous melaphyre—the remnant of the slaggy
scoria which once coated the walls of the volcanic orifice."
The Fifeshire examples, and others where the contiguous
strata are unaltered, find a parallel in the extinct volcanoes
^ "Geol. Mag.," vol. Iii. p. 246.
472
STRUCTURAL GEOLOGY. [PABT II.
of the Eifel, of which Sir Charles Lyell says,^ " The most
striking peculiarity of a great many of the craters above
described is the absence of any signs of alteration or
torréfaction in their walls, when these are composed of
regular strata of ancient sandstone and shale There
is indeed no feature in the Eifel volcanoes more worthy
of note than the proofs they afford of very copious aeriform
discharges, unaccompanied by the pouring out of any
melted matter, except here and there in very insignificant
volume."
Bosses.—These are intrusive masses of igneous rock,
from which the upper parts have been removed, and which
are so far dissociated from any erupted rocks as to make it
doubtful whether they were connected with volcanoes or not.
Many of them are probably the basal parts of volcanic
necks, but others may be parts of intruded masses which
were arrested in their upward progress, and did not suc¬
ceed in reaching the surface. Consequently, it is usual to
speak of all such intrusions as bosses until some evidence of
their true relations can be obtained.
Bosses generally consist sf coarsely crystalline rock, but
sometimes they are partially or wholly composed of com¬
pact rock with a porphyritic structure ; and it is often ob¬
servable that the interior portions are much more largely
crystalline than the outer portions of the mass.
An excellent instance of this came under the writer's
notice at Dalmeny on the Firth of Forth, where the Car¬
boniferous shales and sandstones are interrupted by an
irregularly oval mass of igneous rock more than a mUe
in width. Its outer portion is an ordinary fine-grained
dolerite, but towards the interior the crystals of augite and
felspar became very distinct, and the central portion is as
coarsely crystalline as any granite, some of the augite
crystals being an inch in length ; scattered crystals of
quartz and mica increase the granitoid aspect of the rock,
which may perhaps be designated as an aqgitic gabbro.
They usually appear as irregularly round or oval masses,
forming conspicuous hills, and the stratified rocks around
them are always more or less baked and altered. On a
' "Elements of Geology," eighth edition, p. 674.
CHAP. X.J IGNEOUS ROCKS AS ROCK-MASSES.
473
geological map they appear to take the place of so much
stratified rock, and seem to have cut or bored their way
upward without much disturbing either the strike or the
dip of the strata in which they occur.
The eminence called Barrow Hill, near Dudley, may be
cited as an instance. This mass consists of coarse basalt
or dolerite, and its surface exposure forms a pear-shaped
area (as shown on the Geological Survey map), but below
the surface it widens out in the manner suggested dia-
grammatically in fig. 156, sending out tongues into the
adjoining coal-measures, which are baked and charred.
One of the most striking series of bosses is to be found
in Carnarvonshire, and good evidence has recently been
obtained for considering them to be the basal portions of
the volcanic vents from which were erupted the great
Fig. 156. Intrusive Mass of Dolerite at Barrow Hill, Dudley.
1. Dolerite. 2. Coal-measures. 3. Faults. 4. Altered coals, etc.
series of volcanic rocks that form some of the highest and
most picturesque mountains of North Wales. These
bosses occur at intervals along a line which runs from
Penmaenmawr, near Conway, through the Snowdon range
into the peninsula of Lleyn, a distance of forty miles ; they
include the great domes of Foel Fras and Mynydd Mawr
and the remarkable group of smaller eminences which rise
from the coast near Tr Eifl and Nevin. Their real connec¬
tions were first pointed out by Mr. A. Harker,^ from whose
excellent description, and from the still more recent notice
by Sir A. Geikie ' the following accounts are taken.
The largest and most important of these bosses is that
of Foel Fras, which rises to 3,000 feet above the sea, and
( "The Bala Volcanic Series of Carnarvonshire," Sedgwick
Prhe Essay, 1889.
' "Presid. Address to Geol. Soc.," 1891, p. 125.
Moel Creigian Foe
Eilio. Gleision. Penllithrig. Eras.
I'eiuiiaeuiuaw r.
Fig. 157. View of the Mountains of North Carnarvon, from near Llandudno.
This and Fig. 158 arc reproduced from the " Pictorial Itinerary of North Wales.'
CHAP. X.] IGNEOUS ROOKS AS ROCK-MASSES.
475
forms the centre of the fine group of mountains which
occupy the north-east of Carnarvonshire. A large part of
Poel Eras consists of masses of diabase surrounded by beds
of ash and agglomerate and some sheets of felsite, these
rocks being evidently part of the core of an ancient volcano,
while on the western side is a great plug or boss of porphy-
ritic quartz-felsite which seems to occupy the site of a later
volcanic vent. The rock itself is of an interesting character,
for the ground mass is a micrographie mixture of quartz
and orthoclase, and the porphyritic crystals are plagioclase,
felspar, and augite.
Mynydd Mawr is the bold hill which rises to a height of
Mynydd Maw
Fig. 158. View of Mynydd Mawr from the North.
2,300 feet from the north-west end of Llyn Cwellyn, near
Snowdon, and consists entirely of igneous rock. Its out¬
line, as viewed from the north, is shown in fig. 158, and
another view of it, from the south, will be found in
Part III., Chapter V.
The relations of this protrusion to the surrounding
slates are such as to show that it was intruded before the
original shales were converted into slates, for the cleavage
planes along which they now split change their direction
so as to curve round the mass, while the strike of thebedding
is not altered. This relation, which helps to fix the age of
476
STRUCTURAL GEOLOGY,
[PAET II,
the boss, is shown in fig. 159, which is an enlarged copy of
Mr. Harker's plan.^ Near the borders of the igneous rock
the slates are much indurated and porcellanized, especially
along the northern border of the mass.
The rock of Mynydd Mawr is a micro-granite with por-
phyritic crystals of a peculiar blue variety of hornblende :
it is supposed to be the root of one of the volcanoes which
Fig. 159. Map of Mynydd Mawr.
Scale one inch to one mile.
M, Mynydd Mawr. c, Llyn Cwellyn. F F, Line of fault.
poured out the lavas and ashes that are found in Snowdon,
Lliwedd, Crib Goch, Moel Hebog, and other mountains
to the east of it. These will be described in the next
chapter.
Bosses of igneous and especially of granitoid rock are
frequent both in the Highlands and in the southern up¬
lands of Scotland. Sir A. Geikie describes them as
having a rounded or oval ground plan, and rising into
' " Geol. Mag.," Dec. 3, vol. v. p, 223.
CHAP. X.J IGNEOUS ROCKS AS ROCK-MASSES.
477
dome-shaped hills or prominences ; they pierce the strati¬
fied rocks, seriously interfering with the general dip and
strike. Many of them are flanked by felspathic lavas
and tuffs of contemporary date ; and he therefore regards
them as the stumps of some of the volcanic necks which
supplied the melted and fragmental materials.
Large areas of Granite.—A remarkable series of
granite bosses is found in Devon and Cornwall which seem
to exhibit another stage in the dissection of volcanic
centres. The best known of these is the granite area of
Dartmoor, which occupies a space of about 240 square
miles, and rises to a height of 2,000 feet above the sea.
Notwithstanding its great size, its relations to the sur-
rotmding rocks are similar to those of the bosses above
described.
It penetrates and alters rocks belonging to two different
systems, viz., the Carboniferous and Devonian. Its intru¬
sion, however, has not everywhere brought up the lower
(viz., the Devonian) rocks to the surface ; neither does it
appear to have acted in any sense as a geological axis
or centre of elevation, but it cuts across the boundary line
between the two sets of rocks, breaking through them in a
large irregular mass, which rises into hills of considerable
height and clearly occupies the place of so much stratified
rock (see fig. 160).
Considering the great size of this mass of granite, its
metamorphic action on the contiguous rocks is not very
great ; this does not generally extend to a greater distance
than a quarter of a mile, but varies according to the
nature of the rocks ; and where the beds dip off the
granite, as they do on the north-east side, it extends farther
into them.
It has been suggested that the granite of Dartmoor is
the basal core of an old volcano, and there are facts which
favour this view, but if so, all the superficial and eruptive
portions of the mass have been broken up and destroyed,
and the neck has been worn down to the very base of the
igneous protrusion, so that its present surface is probably
not very far above the general surface of a wide-spread
granitic mass which was the magma or reservoir whence
the Dartmoor material was derived.
478
STRUCTURAL GEOLOGY.
[part ii.
With regard to the possibility of its being the base of a
volcano, it may be observed that if a volcano did exist above
Dartmoor it was almost certainly a terrestial cone, and not
a submarine vent like those of North Wales. From what
we know concerning the geographical conditions of the
periods succeeding that of the Carboniferous, it must long
have remained exposed to the detritive action of atmo¬
spheric agents, and it would be a chance whether any
fragment of it survived; it happens, however, that rocks
g, Granite. D, Devonian, c. Carboniferous.
belonging to one of these periods exist to the east of Dart¬
moor, and they are crowded with blocks and fragments of
felspathic lavas, suggesting by their nature and abundance
that they are the débris of large masses of volcanic
material.'
The other granitic intrusions of Devon and Cornwall
exhibit similar phenomena. There is absolutely no relation
between the boundary lines of the granite bosses and the
^ Consult R. N. Worth in " Quart. Journ. Geol. Soc.," vol. xlv.
p. 398, and vol. xlvi.
CHAP. X.J IGNEOUS KOCKS AS ROCK-MASSES.
479
dip or strike of tke slates into which they penetrate. On
this point Mr. Jukes remarks : " The entire want of con¬
formity between the irregular outline of the surface of the
granite and the position of the aqueous and metamorphic
slates about it has, I believe, been the origin of much
scientific and some practical misapprehension. Mr. Curwen
Salmon was kind enough to send me some time ago an in¬
stance of the latter from one of the Cornish mines, where,
trusting to the apparent dip of the slates, a shaft was sunk
which was expected to be wholly in the slate, but which
unexpectedly came down upon a subterranean mound of
granite. The student will do well to recollect that it is
quite possible for beds to dip directly at and into a mass of
granite." ^
Still larger tracts of granite occur, which cannot be re¬
garded as subsidiary to a single volcano, but seem to have
been extensive reservoirs of molten rock which consolidated
under à great thickness of superincumbent strata ; such
tracts often occupy large districts, and evidently extend to
a great depth, having been exposed by the removal of the
strata which originally covered them.
Granite used frequently to be described as forming the
axis of mountain chains, or the nucleus of mountain-masses,
and it was taken for granted that the upheaval of the
granite tilted and shouldered ofE the stratified rocks which
rested upon it, so that these always dipped away from the
granite; the lowest formation resting against it on each
side with a regular upward succession as we proceeded from
it. The occasional appearance of granite masses in the
centre of mountain chains favoured this view, but more
careful and extended examination showed that this was an
accidental relation.
It is doubtless true that granite is more frequently asso¬
ciated with the older rocks than with the newer, in other
words, with the lower than with the higher rocks. This,
however, is not to be wondered at, seeing that granite is
■essentially a deep-seated rock, and that it has risen from
beneath the earth's crust ; in so doing it must have passed
through the lower rocks in order to reach the higher, and
' Jukes' "Manual of Geology," third edition, p. 245.
480
STRUCTURAI. GEOLOGY.
[part ii.
many injections of granite have only penetrated the lower
rocks without proceeding further.
There is also a still more cogent reason why granite
should occupy this position, viz., that all granite now found
at the surface must be there in consequence of vast denu¬
dation having taken place, great masses of other rocks
having been removed by detrition, together, perhaps, with
much of the granite that once existed above the present
surface. This denudation of course exposes the lower rocks
to view, while parts of the higher rocks that were perhaps
equally penetrated by the granite have been removed, the
other parts' which remain being at a distance from the
granite, and showing no signs of such penetration.
The relations of a large mass of granite to the adjoining
rocks are generally rather comphcated, the boundary line
being very irregiilar, and the granite running out into
tongues and bosses as already mentioned. Where the
junction of granite with other rocks can be studied over a
large area it suggests the arrangement exhibited in the
diagram, fig. 161. The molten rock would seem to have
forced its way upwards and sideways, eating away support
after support of the mass above it, and probably in some
cases actually melting them down and absorbing their
materials into itself. The pressure from below has caused
not only injections of the yet molten rock into the cracks
and fissures of the superincumbent mass, but undulations
in the general surface of the granite, some parts of the
overlying mass being heaved up, and others sinking down
into the granite. The upper surface of the granitic mass,
on its final consolidation, would thus be an excessively
irregular one, with protuberant mounds and deep hollows,
while the beds of the superincumbent mass would not be
likely to conform at all to this irregular surface, but would
often dip directly down on to it, or abut against it in all
kinds of ways, and with any amount of inclination.
Fig. 161 may be taken as a diagrammatic illustration of
a tract of stratified rock invaded by granite, o representing
a mass of granite gradually forced upward into the mass
of slate which had already been disturbed and tilted in
various directions. The horizontal lines, ah, cd, ef, g h,
indicate successive surfaces of denudation. So long as the
CHAP. X.J IGNEOUS ROCKS AS ROCK-MASSES.
481
surface was represented by ab, no part of tbe granite
would be seen, and little metamorpbism would be observ¬
able. If the denudation reached to the surface c d, some
veins of granite would be visible, and these would become
more frequent along the surface ef; intermediate surfaces
between ef and g h would expose thick bosses of granite
with many patches of micaceous schist upon them, and
apparently dipping down into them ; the width of the
granite area would be wide in proportion to the depth of
the denudation, and when a large area was exposed, the
veins would only be visible round its borders.
These ideas respecting the form of the original subter¬
ranean surface of granite masses are corroborated by an ex¬
amination of the districts where large areas of granite are
exposed. The largest surface exposure of granite in the
Fig. 161. Diagram to illustrate the intrusion of Granite
(after Jukes).
British Islands is that of Leinster in Ireland ; this granite
area occupies large parts of the counties Wicklow, Carlow,
and Wexford, being 70 miles long and from 7 to 17 miles
wide. The rocks surrounding this area belong to two dis¬
tinct formations, known respectively as the Cambrian and
Ordovician systems, the former being the lower and older.
Each of these are several thousand feet thick, and their
component beds are greatly bent and contorted. Now
although we are compelled to suppose that the granite must
have come through the Cambrian rocks before it could
penetrate the Ordovician, yet it is remarkable that in no
instance does any part of the lower or Cambrian fon-
mation come in contact with the granite at the surface,
though it does come in some places within two or three
miles of it.
I I
482
STRUCTURAL GEOLOGY. [PAET II.
This, therefore, is a sufficient refutation of the old idea
that the intrusion of granite was the cause of elevation, and
that the rocks in contact with it were always the lowest
and oldest to be found in the country. In the Leinster
district it is Silurian or newer rocks which are seen to be
altered into gneiss and mica-schist by contact with the
granite. " Moreover, in those parts where the granite forms
lofty hiUs, the mica-schist spreads far up on the flank of
those hills, and on the very loftiest, such as Lugnaquilla
(which is over 3,000 feet above the sea), large patches of
mica-schist occur even on the summit, so that the surface
exposure of granite is there narrowest and most inter¬
rupted." Had the hills been left another 500 or 1,000 feet
higher, the granite would apparently have been entirely
concealed there by masses of mica-schist stretching com¬
pletely over it.
" On the other hand, where the granite forms low ground,
as above Tullow and Hacketstown, its surface exposure is
there by far the widest, and all the central part of it is
completely free from patches of mica-schist.
" It is obvious that these differences are the result of the
different amount of denudation that has acted on the
granite. Where the ground is loftiest we have the nearest
approach to the original surface of the granite and its
original covering of other rock ; where the denudation has
cut down deepest, so as to form low groimd, there we get
deeper into the granite mass, or further from its original
surface, to a depth, indeed, to which no mass of mica-schist
could extend, unless it were altogether detached from the
overlying mass, and enclosed in the granite." ^
It is therefore almost certain that the whole of Wicklow
and Wexford is underlain by a continuous mass of granite
with a very irregular surface. The granite district of
the Mourne Mountains in county Down presents similar
features.-'
^ Quoted from Jukes' "Manual of Geology," second edition,
p. 318. It should however be stated that the altered rocks cannot
be properly caUed gneiss and mica-schist, though they may be
described as micaceous schists.
" See "ISIemoirs of the Geol. Survey of Irelánd," Sheets 60, 61,
71, 72, one vol., 1881, and Hor. Section 22.
CHAPTER XI.
igneous kocks eegabded as eock-masses.
b. Veins, Dykes, Sills, and Laocolites.
Connection of Plutonic and Volcanic Rocks.
—From the descriptions given in the preceding
chapter the reader will be prepared to accept the remark
made long ago by Jukes, that " if we could follow any
actual lava-stream to its source within the bowels of the
earth, we should in all probability be able to mark in its
course every gradation, from pumice or obsidian to actual
granite."
The probability of this might be deduced from the simi¬
larity in the ultimate chemical composition of Granites,
Pelsites, Ehyolites, and Obsidians, as shown by the follow¬
ing table of analyses, in which their mean composition is
given :—
Granite.
Q. Felsile.
Ehyolite.
Obsidian.
Silica
72-8
74-27
76-0
71-0
Alumina
15-3
13-12
13-2
13-8
Potash
6-4
4-44
3-1
4-0
Soda
1-4
2-48
4-5
5-2
Lime
0-7
1-17
0-8
1-7
Magnesia
0-9
0-52
0-3
0-6
Oxides of Iron and Manga¬
nese
1-7
2-43
1-7
3-7
Loss by Ignition ....
0-8
1-06
0-6
0-6
100 0
99-50
100-1
100-0
484
STRUCTURAL GEOLOGY.
[PAET II.
Granite Tongues and Veins.—Similar conclusions
may be deduced from a study of the tongues and veins
which proceed from large masses of granite and penetrate
the surrounding rocks. Granite veins, when traced from
the parent mass, generally pass into a fine-grained rock,
which is some variety of micro-granite, felsite, or eurite,
commonly having but little mica, and consisting almost
entirely of a granular or micro-graphic mixture of quartz
and orthoclase. In other cases they lose both quartz and
mica, and appear to take up other elements from the rocks
which they penetrate, so as to become very different in
mineralogical character from the central portions of the
parent mass.
An excellent instance of the passage of granite into a
more basic rock has been described by Professor Haughton.
Near Carlingford, in county Louth, there is a granitic
tract about five miles in diameter, and the composition of
the main mass is as follows :—
This granite sends veins into the overlying Carboniferous
limestone, and the contact of the two rocks—igneous and
aqueous—has greatly altered the characters of both. The
limestone is converted into a bluish sugary marble contain-
Fig. 162. Granite Veins.
Quartz .
Orthoclase
Mica
20-70
66-37
12-76
99-83
CHAP. XI.] IGNEOUS ROCKS AS ROCK-MASSES.
485
ing garnets, -while the granite presents the folio-wing modi¬
fications :—(1) the normal granite passed into a syenitic or
hornblendic granite, and a piece taken about ten yards
from its contact with the limestone had the following
composition :—
Quartz 17'16
Orthoclase 67 "18
Hornblende 15'40
(2) the veins proceeding from this hornblendic granite,
and penetrating the limestone, passed into a kind of basic
diorite or corsite consisting of—
Comparing the last two rocks we see " that the quantity of
hornblende remains almost -unaltered, and that the effect
of the addition of limestone to the melted granite has been
to convert the quartz and orthoclase into anorthite. In this
operation the alkalies of the orthoclase have disappeared ;
the lime being a more fixed base at high temperatures has
altogether displaced the alkalies." ^ It may also be ob¬
served that the corsite contains 24 per cent, less silica than
the hornblendic granite.
Segregation Veins.—^Veins of a compact felsitic cha¬
racter are sometimes to be found in the granite itself, and
contrast strongly -with the surrounding coarsely crystalline
and highly micaceous rock. Instances occur at Killiney,
near Dublin, where, in veins about three feet wide, the gene¬
ral mass of the rock is ordinary coarse-grained, micaceous
granite, "but parts of them suddenly change into fine¬
grained, almost compact rock (eurite), in transverse bands
of irregular shape. These bands look as if they were
subsequent veins injected into the other granite; but
as they are strictly confined to the granite veins, and do
not penetrate the adjacent slates, it is impossible to attri-
99-74
Anorthite
Hornblende
85-84
14-16
100-00
^ "Quart. Joum. Geol. Soc.," vol. xii. pp. 192-198.
486 STRUCTURAL GEOLOGY. [PAET 11.
tute such an origin to them. Moreover, in the adjacent
granite large irregularly-shaped masses of this compact
eurite are to be seen coming in quite suddenly, but with
no resemblance to a subsequently intruded mass.
" Other veins, both in the granite and the adjacent slates,
are doubtless of subsequent origin; but even in these
cases it is probable that they are not of a date long pos¬
terior to the intrusion of the main mass of the gramte. It
is possible that on the first consohdation of the upper por¬
tion of the granite cracks and fissures might take place.
Fig. 163. El van traversing ArgUlaceons Schist, St. Agnes,
Cornwall.
into which injections of the yet molten rock below might
be forced. The upper consohdated part of the granite,
although no longer fluid from heat, might yet retain a
very high temperature—^might, for instance, be red hot, so
that the veins injected into it might be soldered, as it
were, firmly to the walls of the fissures."^
Similar veins occur in many masses of G-ranite, Syenite,
and Diorite, and whenever the material has been analyzed
it has been found to be of a more acid nature than the sur¬
rounding rock, always containing more sihca and a larger
^ Jukes' "Manual of Geology,'' second edition, p. 314.
CHAP. XI.J IGNEOUS ROCKS AS ROCK-MASSES.
487
proportion of potash. This is explained by Mr. Teall as the
effect of progressive crystallization, a portion of the mass
consolidating first, and being of a comparatively basic com¬
position -with low silicated felspars, while the remaining
portion formed a sort of mother-liquor with a more acid
composition.
Thus the syenitic rock of Penmaen Mawr, near Conway,
has a micrographie matrix of quartz and orthoclase, in
which are scattered previously formed crystals of lime and
soda felspars. In the mass occur segregation veins, which
Mr. TeaU describes as perfect examples of micro-pegmatite
with crystals of augite, and he observes that " on comparing
the veins with the i-ock it will be noted that they consist
of the minerals which formed last, namely, augite, ortho¬
clase, and quartz. . . . They are composed of the mother-
liquor left after the separation of the more basic com¬
pounds." ' Thus they appear to be portions of the magma
in which sihca was concentrated, but maintained in a liquid
condition by the presence of steam and water-gas under
pressure, and were injected as veins into that portion of
the mass which had previously consolidated.
This view finds confirmation in the fact mentioned on
p. 340, that when analyses have been made of the crystal¬
line and glassy portions of the same rock, the glassy base
always contains more silica than the crystals, and appears
also to hold more combined water.
Dykes and Elvans.—A dyke is a wall-like mass of
igneous rock which has been intruded along the line of a
more or less vertical crack or fissure. Dykes have already
been mentioned in Part I., Chapter II., but those described
in that connection were chiefly the smaller dykes which
intersect the volcanic cone itself, and not the more deep-
seated dykes which traverse the underlying rocks. When
such dykes are exposed to view by subsequent denudation,
the baked or partially fused condition of the rocks which
form the sides of the lava-filled fissure clearly indicate the
igneous character of the intruded material. The extent to
which the adjoining rocks are thus metamorphosed, varies
from a few inches to several feet.
' Teall, " British Petrography," p. 274.
488
STRUCTURAL GEOLOGY.
[part ii,
Dykes are almost always to be found in the neighbour¬
hood of necks, bosses, and intrusive masses. Where these
consist of granite or syenite, the dykes are generally fel-
stones, using that term in the sense of compact rocks of an
acid character ; where the bosses consist of diorite or gabbro
the dykes are compact greenstones. In Cornwall dykes
are called elvans, the grey elvans being felstones, and most
of them consisting of Quartz-felsite, while the hlv^ elvans
are more basic rocks (diabase, etc.). G-rey elvans are
also found roimd the large granitic area of Leinster, de¬
scribed on p. 481, and they are thus referred to by Jukes :
" They are obviously veins derived from the granite, since
that particular variety of rock only occurs in districts
where granite also occurs, and they are generally more
Fig. 164. Dykes at Strathaird, Isle of Skye.
numerous as we approach the granite. They are often
traceable in nearly straight lines for some miles, although
only a few feet in width, several of them sometimes run¬
ning parallel to each other for such a distance, with inter¬
vals of several hundred yards between them. They often
coincide in strike with the slate or other rocks in which they
lie, though they generally cut obliquely across the dip of
the beds, and sometimes also across their strike. They
often alter the rocks in contact with them ; not, however,
like the larger granite masses, by converting them into
mica-schist, but merely producing a greater induration and
a more minute joint fracture, and a brown ferruginous
tinge, giving them what might be called a burnt aspect." ^
The intersection of stratified rocks by dykes of igneous
rock is illustrated in fig. 164, which represents vertical
Jukes' "Manual of Geology," second edition, p. 316.
CHAP. XI.J IGNEOUS ROCKS AS ROCK-MASSES.
489
dykes of basalt traversing nearly horizontal sandstones ia
the Isle of Skye ; some of these dykes have decomposed
and crumbled away, leaving vertical gaps or trenches
in place of the igneous rock which originally occupied
them.
The north-east coast of Ireland (county Antrim) is simi¬
larly trenched in various directions by dykes of basalt,
which are especially numerous in the country near Belfast.
On the shores of the Lough they form straight causeways,
standing up as vertical walls several feet high, exactly as
if they were artificial quays. The slopes of Cave Hill are
traversed by dykes, some of which can be traced up from
the coast, and are seen in the chalk quarries forming
vertical black seams which cut through the white lime-
Fig. 165. Section of a Dyke occupying a Line of Fault.
stone in a most picturesque manner. Some of these
dykes are probably connected with the sheets of basalt on
the hill-top, but others may not have succeeded in reaching
the surface. They vary in width from 2 or 3 feet up to 20
or 30, and inland they are traceable sometimes for several
miles in straight lines across the country.
Occasionally a dyke has been intruded along a line of fis¬
sure which is also a fault, as in fig. 165, where the basalt,
h, occupies the space between the walls of a fault by which
the coal-measures, a, d, are displaced to the amount of 90
yards. This instance occurs in the Dudley coal-field near
Barrow Hill. The intrusion of the igneous rock has doubt¬
less greatly vsddened the original fissure, for the present
width of the dyke is about 140 yards. A good instance of
490
STRUCTURAI GEOLOGT.
[PAET II.
a dyke coincidiag with a fault and thinning out upward
while the faulted fissure continues, is mentioned in Lebour*s
" Geology of Northumberland" (p. 48).
One of the most remarkable examples of a dyke is that
known as the Cleveland dyke, which runs from the York¬
shire coast south of Whitby for a distance of about sixty
miles in a north-west direction to Cockfield Fell in Durham.
This dyke forms a nearly vertical wall of basalt 50 or 60
feet thick, cutting through all the other rocks which it
meets with, and baking them for a distance of some yards
from its sides.
We have no means of knowing how far such dykes ex¬
tend downward into the earth, but if we could follow them
far enough we should probably find that they proceeded
Fig. 166. Slieet of Igneous Rock, a b, traversing Shales, c, and
sending off a Tongue, t.
from some large intrusive mass of diorite or syenite, and
that they exhibited every gradation from the characters of
a Volcanic to those of a Plutonic rock.
Intrusive Sheets, or Sills.—When, instead of filling
a more or less vertical fissure, the igneous rock has been
intruded between the planes of stratification, it is called a
sill. The rocks above and below a sül are, of course,
baked and altered just in the same way as the rocks on
each side of a vertical dyke.
Intrusive sheets of Diabase have been traced for many
miles in the rocks of North Wales, running regularly be¬
tween two beds, as if they were interbedded with them,
till at length they were found to cut obliquely up or down,
and continue their course between other beds (see fig. 166).
CHAP, Xl.j IGNEOUS KOCKS AS ROCK-MASSES. 491
Mr. Harter observes that " these sheets have everywhere
partaken of the contortions of the strata among which they
he, and it is even possible in the case of the larger intru¬
sions to trace the manner in which the folding has been
modified by the presence of these stubborn masses of rock
among less resisting materials. It may be asserted with
some confidence that the latest of the basic sheets were in¬
jected almost at the time when the plication of the strata
was in progress, so that their form and distribution are
partly determined by the folds.^ Occasionally they thicken
out into lenticular masses, but still keep to the same stra-
tigraphical horizon.
The great " Whin siU " is a sheet of enstatite-dolerite,
thrust into and between the Lower Carboniferous strata,
and its outcrop extends for a distance of about 80 miles
through Cumberland and Northumberland, and an inlier
Fig. 167. Dyke and Sheet of Basalt, Trottemish, Skye.
of it occurs in Teesdale, so that it must have a very wide
subterranean extent. Its average thickness is from 80 to
100 feet.
The island of Skye has already been mentioned as ex¬
hibiting many examples of vertical dykes, and associated
with these there are equally interesting and instructive
cases of intruded sheets. Fig. 167 is copied from a sketch by
Dr. Macculloch, and shows a large dyke-like mass of basalt,
a, piercing the sandstones, e, and giving off a thick hori¬
zontal sheet, which splits up into three smaller sheets or
veins (c c c) ; other dykes and veins run into or from this
sheet, both upwards and downwards.
The extent and thickness of intrusive sheets is some¬
times very considerable ; that of the " Whin sill " has
already been mentioned ; in South Staffordshire a large
sheet of dolerite, varying in thickness from 20 to 60 feet.
1 " The Bala Volcanic Series of Carnarvon," p. 76.
492
STRUCTURAL GEOLOGY.
[PAET II.
was found to spread over an area of at least twenty square
miles ; in one part of the district it lies at a certain depth
below the bottom coal, but in another part it passes up¬
wards through that coal and spreads out over it, in some
places even sending up dykes and protuberant bosses into
still higher measures.
In such cases as these we may suppose that having been
forced up through previously-formed fissures to a certain
height, the molten rock then met with such resistance
above, that it was as easy for it to lift the overlying beds
as to break through them (see fig. 167). The planes of
stratification would then become those of least resistance,
and the molten stream would be injected along those which
yielded the most readily, thinning and swelling out accord¬
ing to the varying pressures which were encountered in its
course.
Laccolites.—The explanation just given leads us to the
consideration of the still more massive intrusions, forming
huge lenticular masses of felspathic or trachytic rock, to
which Mr. Gi. K. Gilbert has given the name of laccolites,
or stone-cisterns.^ These appear to consist of the less
fluid acid lavas, which do not flow so rapidly (see p. 26),
and would not be able to make their way between the
planes of stratification so easily as the more fluid basaltic
lavas. Such intrusions, therefore, would exercise still
greater upward pressure, and could not fail to lift the
overlying beds to a greater extent than the doleritic intru¬
sions, while the accumulated weight will doubtless also
depress the underlying rocks to some extent (see fig. 168).
The intrusion will eventually take the form of a vast lenti¬
cular mass or reservoir of igneous rock.
Laccolites are usually drawn with a vertical pipe, or
feeder, beneath them, but it seems probable that in
many cases the feeder was at one end of the lenticular
mass. When they occur in a series of inclined and
flexured strata their outcrops -will form oval or lenti¬
cular patches, the longer axes of which will always coin¬
cide with the strike of the beds in which they be. Mr.
Harker observes that " this elongated form of outcrop and
"Geology of the Henry Mountains," U.S. Geol. Survey,
Washington, 1877.
CHAP. XI.] IGÎS^EOUS ROCKS AS ROCK-MASSES.
493
parallelism with the local strike are in themselves the
strongest circumstantial evidence that the intrusion is a
true laccolite, and indeed admit of no other general expla¬
nation. There is no conceivable reason why an igneous
mass should have its greatest extent of outcrop in the
direction of strike unless the mass be intruded along bed¬
ding planes. Such a mass may, of course, break through
the strata in some places, and a laccolite in direct connec¬
tion with a small transgressive boss of similar rock may
he expected as a not unusual phenomenon.^
Moel Perfedd near Nant Pfrancon is now recognised as a
laccolite, and Mr. Harker regards the neighbouring intru¬
sion of Bwlch Cynion as an instance of a compound lac-
cohte and transgressive boss.
Laccolites sometimes occur in the arches of large flexures
Fig. 168. Diagram of a Laccolite.
in such a manner as to suggest that the liquid material
was intruded during the production of the folds, accumu¬
lating where the tension was greatest, and where the in¬
flowing hquid would be assisted by the lateral pressure in
pushing up the strata. Such a laccolite may be dissected
by erosion, and the separated portions will look like inde¬
pendent intrusive masses.
c. Inteebedded Lavas and Ashes.
Hitherto we have dealt entirely with intrusive masses of
igneous rock ; we have now to consider the arrangement of
those materials which flowed as lavas or were carried
as dust and ash to such a distance from the volcanic orifice
as to be more or less dissociated from the volcanic rock or
centre (see fig. 152).
' " The BaJa Volcanic Series," Cambridge, 1889, p. 43.
494
structural geology.
[part ir.
It has frequently happened that the streams of lava
"which were poured out of a volcano on to the floor of
a neighbouring sea or lake have been preserved, while the
volcano itself has been destroyed. The volcano may have
been situated near the sea-shore, or it may have been a
volcanic island, or it may have been entirely submarine,
and it is the accident of its having been adjacent to an
area of deposition that has led to the preservation of its
lavas.
Each flow or coulee of lava has been covered up by the
deposits which were accumulating in the adjacent lake or
sea, and when the whole area has been upheaved and dis¬
sected by the scalpel of aqueous erosion, the lava-flows
are seen to be interbedded or interstratified "with the sedi¬
mentary deposits. Their very nature and mode of origin,
therefore, limits them to this one form or mode of occur¬
rence, and all such interstratified masses may consequently
be described as sheets. Further, it is evident that they and
the sedimentary beds with which they are associated must
constitute one consecutive series, and that in this sense the
sheets of igneous rock are contemporaneous "with the beds
of stratified rock. With them are frequently associated
deposits of ash or tuff, the material being sometimes fine,
sometimes coarse or conglomeratic, and often compacted
and indurated into a hard and rough rock.
Proofs of Interbedding.—It is particularly important
that the observer should be able to decide whether any
sheet of igneous rock he may meet with is interbedded or
intrusive ; whether, in fact, it is to be regarded as contem¬
poraneous with the rocks among which it lies, or as having
been intruded between them at some subsequent time, for
upon this determination depends the geological date which
is assigned to the rock in question. It -will therefore be
desirable in the first place to examine the signs and circum¬
stances which sei-ve to distinguish interbedded from intru¬
sive sheets of lava or trap. These are,
1. The slaggy or vesicular character of the upper and
lower surfaces of the sheet.
2. The rmaltered character of the overlying beds and
their general relations to the surface on which they rest.
3. The association of stratified ashes or tuffs.
CHAP. XI.] IGNEOUS ROCKS AS ROCK-MASSES.
495
1. In the description of lava-streams given on p. 26, it
was stated that their upper surface or crust was sometimes
rendered slaggy and ropy by the motion of the current, and
in other cases it became vesicular and scoriaceous from the
escape of the contained gases. The surface of all lava-
streams presents one of these two conditions in a greater or
less degree, but intrusive sheets which have solidified under
the pressure of the overlying beds do not exhibit these ap¬
pearances. The under-surface of a lava-stream is also very
frequently full of vesicles, so that it is then vesicular both
at the top and bottom ; subsequent infiltrations may con¬
vert these vesicles into amygdaloids, but this only renders
them more conspicuous. The cavities are often drawn out
and elongated in the direction of the flow, so that by this
means it is sometimes possible to ascertain the direction
from which the amygdaloidal rock has come.
Fig. 169. Lava resting on Sandstone.
2. It is obvious that the beds overlying a contempo¬
raneous sheet cannot be altered by it, because they were
not there when the rock was poured out as a stream of
lava. An interbedded sheet, therefore, only alters and
bakes the beds below, while an intrusive sheet affects those
above as well as those below. The absence or presence of
intrusive tongues or veins proceeding from the sheet is like¬
wise a criterion, their presence at once indicating its intru¬
sive nature. Farther, the stratification of the beds over¬
lying an interbedded sheet generally affords evidence of
their having been deposited upon the irregular surface of
the trap, as indicated in fig. 169. The stratified rock fills
up any hollows or cracks which may exist in its surface,
and ofteii encloses small pébble's or blocks which have been
broken off from it.
3. The presence of stratified tuffs also raises the pre¬
sumption that the associated sheets of lava are interbedded
496
STRUCTURAX GEOLOGY.
[PAET II.
and contemporaneous, thougli it does not absolutely prove
this to be the case, for as we have seen, masses of tuff and
agglomerate frequently fill up volcanic necks, and are then
associated with the intrusive dykes which proceed from
it. Still, if the sheets of trap are clearly interstratified
with beds of " ash," and if the ash-beds do not exhibit any
signs of baking or alteration where they are in contact
with the lava, then it becomes almost certain that the trap
is contemporaneous.
Consecutive Flows.—Occasionally a single flow or
sheet of trap is met with, and sometimes several such single
flows are interbedded with a series of stratified rocks ; more
frequently, however, what appears to be one massive sheet
is in reality made up of several distinct lava-flows poured
out at separate intervals from the volcanic vent. The in¬
dependence of the flows is sometimes indicated by diffe¬
rences of texture, one being compact, another porphyritic,
another columnar, and so on ; even when all are similar in
character, there are sometimes zones of vesicular or amyg-
daloidal rock, formed by the union of the upper and lower
surfaces of two flows, which mark the lines of division be¬
tween them. Lastly, the fact that such a mass is really
made up of successive flows is often clearly demonstrated,
when their outcrops are traced for some distance, by the
intercalation of other beds between some of the component
sheets. Such an occurrence proves that the lava currents
invaded an area where deposition was going on, and that
sufficient time elapsed for material to be deposited on the
surface of the first sheet before it was overflowed by the
second.
An excellent instance in which consecutive flows of dole-
ritic lava are interstratified with shales and sandstones be¬
longing to the Coal-measure series occurs in the Borrows-
tounness coalfield, Edinburgh. The middle part of this
series is described as consisting of the following alterna¬
tions of igneous and aqueous rocks :—
Feet.
Shales and sandstones —
Dolerite 20
Shale and sandstone 6
Dolerite in several sheets, with partings of shale and
sandstone in places 265
CHAP. XI.J IGNEOUS ROCKS AS ROCK-MASSES.
497
Feet.
Shale, coal, and sandstone 30
Dolerite in several sheets 110
Shales and sandstones 25
The " red coal " seam 3
Dolerite
Sandstone 16
In one shaft the thickest compound mass of dolerite was
found to he separated into two portions by the intercala¬
tion of 30 feet of shale and sandstone.^
Such intercalations and alternations of lava-flows and
aqueous rocks are frequent wherever extensive eruptions
have taken place, and the whole groups of strata thus
formed have, of course, partaken in all the accidents of
tilting, flexure, and fracture, by which the district has
been affected since the time of their formation.
Instances of Interbedded Lavas and Ashes.—
1. North Wales.—The ancient volcanoes of Carnarvonshire
have been mentioned on p. 473, and two of them, T-foel-fras
and Mynydd Mawr, have been described. Writing of this
district Sir A. Geikie remarks, " So great has been the
denudation of the region that the pile of lavas and tuffs
which accumulated around and above these orifices has
been entirely swept away. No trace of any portion of that
pile has survived to the west of the line of bosses ; while
to the east, owing to curvature and subsequent denudation,
the rocks have been dissected from top to bottom, until
almost every phase of the volcanic activity is revealed." '
The thickness of volcanic material exposed on Snowdon
alone is estimated at over 3,000 feet, and according to the
most récent observations, they consist of the following
members :—
Feet.
Grey amygdaloidal lavas with a sheet of felsite . 150
Thick beds of tuff with sheets of felsite . . . 1,200
Porohyritic felsites, many of the sheets showing
slaggy surfaces 1,700
" But this includes only the higher part of the whole
volcanic group. Below it come the lavas of Y-Glyder Fach,
"Geology of Edinburgh," Mem. Geol. Survey, p. 63.
' "Pres. Address to Geol. Soc.," 1891, p. 83.
K K
498
STRUCTURAL GEOLOGY.
[part II.
which according to the Survey measurements ' are about
1,600 feet thick, while still lower lie the ancient coulées of
Carnedd Davydd and those that run north from the vent
of Yfoel-fras, which must reach a united thickness of many
hundred feet. We can thus hardly put the total depth of
volcanic material at a maximum of less than 6,000 to
8,000 feet. The pile is, of course, thickest round the vents
of discharge, so that no measurement, however carefully
made, at one locahty, would be found to hold good for
more than a short distance." (Geikie, loc. cit., p. 83.)
From the volcanic centres the lava-flows thin in all
directions, becoming intercalated with beds of shale and
sandstone, and finally dying out altogether. The same
is the case with the ash-beds, which near the volcanic
centre are extremely hard and compact, and consist entirely
of the débris of slaggy and scoriaceous lavas, so that they
sometimes resemble the decomposed portions of the fel-
stones ; but as they are traced from the central area they
become thinner, more regularly bedded, and more obviously
mechanical, the ashy and cindery materials become mixed
with an increasing proportion of sandy, clayey, or calca¬
reous sediment, and they finally pass into pure sandstones,
shales, or limestones.
Remarking on the frequency of slagginess and flow
structure in the lower felsitic lavas of Snowdon, Sir A.
Geikie points out that careful observations on this struc¬
ture would often indicate the direction from which the
lavas came. He instances one of the lowest sheets which
forms a line of picturesque crags on the slope facm^
Llanberis. "The general push indicated by [the curled
and overfolded layers of the lava] points to a movement
from the westward. Turning round from the crags and
looking toward the west, we see before us on the other side
of the deep vale of Llyn Cwellyn, at a distance of little
more than three miles, the great dome-shaped Mynydd
Mawr, which, there is reason to believe, marks one of the
orifices of eruption " (loc. cit., p. 85).
2. The Limemck Basin.—Ireland presents us with a good
example of interstratified aqueous and igneous rocks in
' See Sir A. Ramsay's "Geology of North Wales," Mem. GeoL
Survey.
CHAP. XI.J IGNEOUS ROCKS AS ROCK-MASSES.
county Limerick, whick is quite as in¬
teresting and instructive as tkat of North
Wales, and is much more easily examined
because it has been less disturbed. The
aqueous rocks, too, are chiefly limestone,
so that there is no difficulty in distinguish¬
ing them from those of igneous origin.
In the north-eastern part of Limerick,
enclosed by the lofty ranges of the Keeper,
Slievragh, Knockfeerina, and G-altee Hills,
is a rich plain, the centre of which is diver-
sifled by groups of less lofty, but often
steep and rough-looking hills. These inner
hills rise into craggy knolls some 600 or
700 feet high, while much of the country
about them consists of alluvial flats not
more than 100 feet above the sea. They
consist partly of masses of porphyritic fel-
stone, some red, others purple, and partly
of greenish or blackish rocks which are
more or less altered dolerites and basalts ;
some of the sheets are regularly and beauti¬
fully columnar, others are quite vesicular
and scoriaceous ; the latter generally form¬
ing definite layers between the bands of
compact lava, and indicating the union of
the upper and lower surfaces of different
lava-streams. In many places the vesicles
are filled with amygdaloids of calcite,
quartz, or a zeolite.
Associated with these lavas are large
irregular deposits of ash, consisting of beds
of coarse and fine-grained materials ob¬
viously derived from the lavas. Some of
these beds are coarse conglomerates, con¬
taining rounded blocks of trap and lime¬
stone, some of which are as large as a
man's head, and many as large as the fist.
Others consist of small grains, which vary
in size from that of a pea down to pin-
heads and less, arranged in very regular
500
STRUCTURAL GEOLOGY. [PABT II.
parallel laminae, the coarse and fine layers often alternating
in regular bands about half-an-inch in thickness.'
Some of the sheets of felstone, 800 or 1,000 feet thick,
are found, when followed along their strike, to split up and
let in alternations of beds of ash and beds of limestone,
showing that the greater uninterrupted masses were in
reality formed by successive flows of molten rock at the
bottom of the sea, and that, where each of these flows
terminated or became thin, accumulations of ash or lime¬
stone took place on the sea-bottom in the intervals be¬
tween the outpouring of one flow and that of the next, so
as to cause these interstratifications. Pallas Hill is a con¬
spicuous example of this occurrence.
On the south side of this volcanic area there are five in¬
trusive bosses of felspathic rock rising up through the
lower limestone, and another boss at about the same dis¬
tance on the the north side of the basin. These look like
some of the volcanic foci from which the lavas were de¬
rived, the old roots, as it were, of the submarine lava
flows, exposed to view by the denudation of the limestones
and traps that once covered them. Other foci or intrusive
masses are doubtless concealed beneath the existing beds
of trap in the central parts of the basin.
Professor Hull more recently has distinguished two
epochs of eruption in the Limerick area, the first being an
extrusion of felspathic lavas and ashes, while the second,
which occurred after a considerable interval, gave birth to
flows of augitic lava (basalt, etc.) The sect. (fig. 170),
copied by permission from Prof. Hnll's " Physical Geology
of Ireland " illustrates the succession of materials erupted
during the earlier outburst.
3. Scotland.—The country which lies on either side of the
Firth of Forth was anciently the scene of great volcanic
activity. Arthur's Seat in Edinburgh and Largo Law in
Fife are excellent instances of denuded volcanic piles ; the
numerous necks of Fifeshire have already been mentioned,
and the behaviour of interbedded and intrusive sheets of
' See the Explanations of Sheets 143, 144, 153, 154 of the Geolo¬
gical Survey of Ireland, and Jukes' " Manual of Geology," second
edition, p. 325.
CHAP. XI.J IGNEOUS ROCKS AS ROCK-MASSES. SOI
lava can be studied in the cli£E sections along the coast. ^
Most of these eruptions took place towards the end of the
Carboniferous period, but evidence of an earlier series of
Tolcanic explosions are to be found in the ranges of the
Sidlaw, Ochil, and Pentland Hills. " AH these prominent
ridges," says Sir A. Geikie,^ " are formed mainly of vol¬
canic materials belonging to the period of the Lower Old
Eed Sandstone. Their general characters—beds of lava
(porphyrite, etc.) with sheets of tuff and interstratified
sandstones, shales, and conglomerates—^leave no doubt
whatever as to their having been truly erupted to the sur¬
face during that geological period.
Basaltic Plateaux.—It has long been assumed that
all erupted basaltic sheets were remnants of lava-flows
which had once been connected with true volcanic cones.
Baron Eichthofen had indeed pointed out so long ago as
1868, that on the Pacific slope of the Eocky Mountains
there is evidence of the emission of vast floods of lava
without the formation of cones and craters, and he stated
his belief that these eruptions had proceeded from great
fissures out of which the lava had poured in a continuous
stream. This theory was energetically opposed by Mr.
Poulett Scrope, who demonstrated that the extensive
plateaux of basalt in Auvergne were only separated from
the cones with which they had originally been connected in
consequence of the great erosion to which that country had
been subsequently exposed. He argued that the apparent
isolation of other plateaux was merely owing to the total
destruction of the volcanoes from which they had been
emitted (see Plate, fig. III).
Subsequent observations, however, have tended to con¬
firm the probability of Eichthofen's views ; the extent of
these American lava-floods has been found to be enormous,
and it is estimated that in the States of Oregon, Idaho,
Nevada, and Northern California they cover an area of
200,000 square miles. The date of their emission is com¬
paratively recent ; they have been poured forth over the
bottoms of the present valleys, and the only natural sec-
' Consult Sir A. Geikie on the " Volcanic Rocks of the Basin of
the Forth, Trans. Roy. Soc. Edin.," xxix. p. 437.
" " Trans. Geol. Soc. Edin.," vol. ii. pt. lii.
502
STRUCTURAL GEOLOGY.
[PAET II.
tions of them are those in the gorges or cañons which the
present rivers have been able to erode out of the vast plain
of basalt.
Sir A. Geikie thus describes one of these enormous lava
fields ' :—" We had been riding for two days over fields of
basalt among the valleys, and [at last] emerged from the
mountains upon the great sea of black lava which seems to
stretch ilhmitably westwards. It was as if the great plain
had been filled with molten rock which had kept its level,
and wound in and out along the bays and promontories of
the mountain slopes as a sheet of water would have done.
.... I looked round in vain for any central cone from
which this great sea of basalt could have flowed. It
assuredly had not come from the adjacent mountains,
which consisted of older and very different lavas round
the worn flanks of which the basalt had eddied. ... I
became convinced that all volcanic phenomena are not to
be explained by the ordinary conception of volcanoes, but
that there is another and grander type of volcanic action
where the molten rock has risen in many fissures, and has
weUed forth so as to flood the lower ground with successive
horizontal sheets of basalt."
He suggests that the similar sheets and dykes of basalt
which occur in the north of Ireland and west of Scotland,
and the enormous dykes which are so frequent throughout
Scotland and northern England, are all manifestations of a
grand series of fissure eruptions comparable to those in
North America, but belonging to an earlier geological
period. In support of this conclusion he appeals to the
wide extension and horizontality of the basalt sheets, the
absence or paucity of inter stratified tuffs, and finally thei
apparent connection with a series of lava-filled fissures, some
of which are no less than 200 miles distant from them.
It should be mentioned, however, that the cogency of
this evidence is disputed by Prof. Judd, and that con¬
temporaneous volcanoes from which the lavas might have
come certainly existed both in Scotland and Ireland.
That portion of the great lava-flood which is isolated in
the north-east of Ireland covers almost the whole of county
' " Geological Sketches," by A. Geikie, p. 278.
CHAP. XI. J IGNEOUS ROCKS AS ROCK-MASSES.
503
Antrim with a mass which is in some places 900 feet
thick ; it is 50 miles long by 30 wide, or about 1,200
square miles in area. The basalt mass consists of numerous
sheets or flows, some of which are quite amorphous, either
compact or amygdaloidal, while others are beautifully
columnar ; one of the columnar sheets dipping gradually
into the sea on the north coast is known as the Giant's
Causeway.
Similar extensive plateaux of basalt occur in other parts
of the world, in Abyssinia, western India, and Victoria,
and probably mark the sites of some of the great fissure-
eruptions which have taken place at different times in the
history of the world. In their recent résvmé of the
geology of India, Messrs. Blandford and Medlicott de¬
scribe the persistent horizontality of the great basalt sheets
which form the plateaux of the Deccan, the absence of any
trace of associated volcanic cones, and the abundance of
dykes which traverse the underlying platform of older
rocks, where these emerge from beneath the basalt-covered
area.
CHAPTEE XII.
MBTAMOEPHIC EOCKS,
HE consideration of these rocks has been left till this
stage because the peculiar structures which they
exhibit could not be properly explained to the student
until he had been made acquainted with the manner in
which the unaltered rocks, both of aqueous and igneous
origin, occur in the earth's crust, and had obtained some
idea of the results caused by the intrusion of igneous
among aqueous rocks, and by the earth-pressures to which
both have been subjected from time to time.
Metamorphic rocks may be broadly defined as those
which, after their first consolidation, have been so altered
that their internal structure and their physical characters
are different from what they originally possessed. This
change may be molecular, or atomic, or may be chiefly the
result of mechanical stress or pressure ; thus : (I) The
molecular arrangement and physical character of the
rock may be altered without much mineral change; for
this kind of metamorphism Dr. Irving has proposed the
name of metatropy. (2) The original mineral particles of
the rock-mass may have been rearranged by mechanical
stress ; for this kind of change Dr. Irving has suggested
the term metataxis.^ (3) The mineral components of the
rock may be recombined into minerals of different chemical
composition; for this Prof. Bonney has suggested the excel¬
lent term metacrasis.'
A simple case of metatropy is that of a limestone which
^ " Metamorphism of Kocks," Longmans, 1889, p. 5.
" "Presidents Add. GeoL Soc.," 1886, p. 31.
CHAP. XII.] METAMORPHIC ROCKS.
505
has been converted into a crystalline-granular marble by
the heat of intrusive igneous rock, its optical and physical
characters being changed while its chemical composition
remains nearly the same.
A simple case of metataxis is that in which a rock has
been so compressed that its component particles are all
elongated in one direction, and the rock has consequently
a tendency to split in that direction. A rock which has
this structure and splits into thin layers which are not
those of original deposition is said to be cleaved, and the
structure is called cleavage, or slaty-cleavage, from its lead¬
ing to the production of slates.
Metacrasis may be sometimes of a simple character,
either paramorphic, as in the change of a dolerite into an
epidiorite by the conversion of augite into hornblende, or
pseudomorphic, as in the production of serpentine from an
olivine rock ; but it is often a more complicated process, as
in the combination of the disintegrated mineral particles of
a sedimentary rock and their recrystallization into fresh
minerals. It must be remembered that clays and clayey
sandstones consist of minute particles of kaolin, mica, and
quartz-sand, derived from the disintegration of granitic
rocks, and that they usually contain small amounts of iron
oxide, lime, and alkali ; so that under the combined influ¬
ence of pressure and superheated water-vapour these
particles might be recombined into crystals of felspar,
mica, and quartz, like those from which the ingredients of
the clay were originally derived.
When in the course of such recrystallization the minerals
have arranged themselves in thin lenticular layers which
are all elongated in the same direction, so that the rock
splits along them, the rock is said to be foliated (from
folium, a leaf) and the structure is termed foliation. This
term was originally proposed by Professor Sedgwick, and
was defined by him as " a separation into layers of different
mineral composition," while cleavage means only " a ten¬
dency to split in a mass of the same composition." Folia¬
tion is a chemical change effected under the concomitant
influence of great pressure, and the structure may be pro¬
duced in crystalline igneous rocks as well as in clastic
sedimentary rocks. A kind of fohation may also be pro-
506
STRUCTURAL GEOLOGY. [PAET II.
duced in an igneous rock by fluxional movements in the
mass before it was completely solidified and still in a
viscid condition.
It must be borne in mind that many masses of igneous
rock may be included in a region that is being compressed
and metamorphosed, and that the alterations effected in
the igneous and aqueous rocks will tend to make them
resemble one another much more than they did in their
unaltered states, so that it may sometimes be difficult to
say whether a given foliated rock was originally an aqueous
or an igneous formation.
There are certain special structures exhibited by meta-
morphic rocks which must be defined before the rocks are
described. The following are the most important.
When the rock consists of small even-sized crystals of
quartz and felspar it is said to be granulitic, and there
may be a granulitic ground-mass with lenticular lumps or
phacoids consisting of larger crystals.
When a rock has been crushed so that it consists of
small lenticular or granular components separated by a
fine-grained material, which appears as a " flowing tissue
of opaque fibres and strings " (Lapworth), as if the rock
had been partially ground to dust between millstones, it is
said to have a mylonitic structure, and is sometimes called
a mylonite.
When the foliated materials are arranged around some
hard crystal, so that this, with the curving layers above
and below it, resembles an eye, we have what is called the
" eye structure " (aagfew-structure).
When the matrix looks as if it were flowing round and
through the larger components, the rock is called, by the
Germans, flaserig, or veiny, because it always has a veiny,
banded appearance to the eye. This arrangement is often
spoken of as flaser-structure, and it may exist apart from
eye-structure in a rock.
The following are brief descriptions of the different
kinds of metamorphic rocks which are generally recog¬
nized. Among the less altered forms, those of aqueous
and igneous origin can be easily distinguished, but among
the more altered rocks this becomes a difficult matter.
CHAP. XII.] METAMORPHIC ROCKS.
507
A. Rocks without Foliation.
Under this head are included (1) sedimentary rocks
which have been altered by heat only (baked rocks), or by
pressure which has not been carried beyond the degree
necessary to produce cleavage; (2) igneous rocks which
have undergone simple paramorphic changes.
1. Altered Sedimentary Boeles.
Quarteite.—This is an altered sandstone, and has been
defined as a rock consisting of quartz-grains cemented
together by crystalline quartz, the crystals of which are
commonly arranged in crystalline continuity with the quartz
of the original sand-grains. It is thus distinguished from
sandâtones with an infiltrated siliceous cement, which is
always either opaline or chalcedonic. It is probable that
the secondary quartz in quartzite has been derived from
the sand-grains by the solvent action of hot vapour or
steam, for it must be remembered that every intrusion of
igneous rock must generate such vapour out of the inter¬
stitial water in the surrounding stratified rock.
Halleflintas are compact siliceous rocks, not unlike fel-
stones in general aspect. They may have been formed from
fine-grained felspathic sandstones by induration and par¬
tial fusion, but some may have resulted from the alteration
of fine felspathic tuff (trachytic dust).
Porcellanite and Argillite are baked and indurated clays,
appearing as close-grained flinty or jaspery rocks, generally
of a greenish, greyish, or reddish colour. Lydian stone
is a black or dark-brown rock of similar texture, resulting
from the metatropic alteration of a carbonaceous clay
or shale; it often contains crystalline grains of quartz,
and must then have been either a sandy shale or a sili¬
ceous clay like fire-clay.
Marble is a limestone altered by heat and pressure into
a crystalline rock consisting of closely-packed crystals of
calcite. The limestone may have been origiaally earthy,
shelly, or partly crystalline ; by marmorosis (as the making
of marble is termed) all the carbonate of lime is recomposed
508
STKUCTURAL GEOLOGY. [PAET II.
into crystalline grains of calcite, which are interlocked one
with another.
By heating chalk and limestone in closed tubes, to prevent
the escape of carbonic acid gas, Sir James Hall succeeded
in converting them into hard crystalline marble, and in
another experiment he actually fused the substance, which
then acted powerfully upon other rocks. In Nature, heated
water under pressure of superincumbent rocks has pro¬
bably played a prominent part in the production of marble.
The chalk of the north-east of Ireland, where penetrated
by dykes of basalt, is altered in some places into a hard
grey semi-crystalline limestone, and in others into a white
coarsely-crystalline marble. In many marbles certain sili¬
cates, such as idocrase, garnet, and green augite have been
developed, and in some mica and graphite (pure carbon)
has been found.
Ophicalcite is the name for a group of rocks which requires
further investigation ; it may be defined as consisting of
crystalline calcite or dolomite, veined and permeated with
serpentine. Some ophicalcites, like the Galway serpentine
or Connemara marble, certainly seem to be metamorphic
limestones in which lime-sihcates have been formed, and
subsequently altered into serpentine ; but others may be
only broken serpentines recemented by infiltrated calcite ;
and others again may possibly be due (as suggested by
Professor Or. Cole) to the complete decomposition of an
igneous rock consisting of anorthite and olivine.
Lime-silicate roch (Kalkhomfels).—Limestones in con¬
tact with large masses of igneous rock have undergone a
remarkable degree of metamorphism, resulting in complete
paramorphic change. The carbonic acid seems to have
been gradually driven off by the heat, and its place has
been taken by silicic acid, with which the lime has com¬
bined to form various lime silicates ; so that the rock has
been changed from a calcareous to a calcáreo-siliceous one,
such as the Germans call Kalkhomfels, or calcareous hom-
stone. It often happens that a limestone contains some
argillaceous matter which enables an alumino-calcareous
silicate, such as augite, to be formed.^
^ See Harker and Marr on Shap-Granite, " Quart. Joum. Geol.
Soc.," vol. xlvii. p. 314.
CHAP. XII.] METAMORPHIC ROCKS.
509
The gradual development of the silicates at the expense
of the calcite is often traceable where a limestone can be
followed along its strike till it abuts on the igneous rock.
At a certain distance from the latter the limestone is only
a marble with a few small scattered crystals of some lime-
silicate ; nearer to it these crystals are larger and more
numerous ; nearer still they form the greater part of the
rock, only a few patches of calcite being left ; finally, within
300 yards of the igneous mass, the rock consists entirely
of silicates, and becomes either a light brown idocrase-
gamet rock, or a pale bluish or greenish grey homstone,
consisting chiefiy of tremolite or pyroxene. These changes
are exhibited by the Conisten limestone near its contact
with the Shap-Grranite (Westmoreland).^
Slate is a fine-grained argillaceous rock in which cleavage
has been developed, so that the rock splits into thin
parallel layers, which are more or less oblique to the
planes of bedding. Slates are generally cleaved clays or
shales, but some have been formed from fine volcanic tuff.
The original lamination is generally imperceptible, but in
some slates it is marked by bands of different colour, and
these are termed the stripe of the slate. When tracts of
slate are examined in the field the original bedding is
often indicated by alternating bands of fi^ner and coarser
slate, which represent the beds of different texture in the
originally stratified rock.
PhylUte is a slate in which some secondary mica has
been developed, this mica appearing in minute silvery
flakes along the planes of cleavage, and giving a peculiar
glossy aspect to the slate. In addition to this, phyllites
frequently exhibit a minute wrinkling, which may be re¬
garded as an incipient stage of schistosity or foliation.
In the neighbourhood of igneous masses slates often show
signs of further chemical metamorphism in the occurrence
of sporadic crystals of chiastolite, andalusite, garnet, and
other silicates of alumina.
' See Messrs. Harker and Marr's paper, " Quart. Journ. Geol.
Soc.," vol. xlvii. p. 310.
510
structural geology.
[part ir.
2. Altered Igneous Rocks.
Altered Granites.—Along the borders of a granitic mass
accessory minerals are often developed, due probably to
the subsequent upward passage of steam and hot spring-
waters. One of the commonest of such minerals is Schorl
or Tourmaline, which seems to be developed at the
expense of the mica or felspar. The granite is then termed
schorlaceous. When the rock is entirely composed of
quartz and schorl without felspar it is called Schorl-rock.
A beautiful variety of schorlaceous granite, where crystals
of red orthoclase are scattered through a matrix of quartz
and schorl, occurs at Luxullian in Cornwall, and is known
as Luxulianite.
Sometimes chlorite, cassiterite (tin-ore), and pyrites are
added to the constituents of granite, forming a blackish
rock, which is called Zwitter. A rock consisting of quartz
and lepidolite, with scattered crystals of topaz, developed
at the expense of the felspar, is called Greissen ; it occurs
in Saxony, but is not of common occurrence.
Altered Gabbros and Bolerites.—The felspar of these
rocks has often been converted into a hard white mineral,
which was formerly called Saussurite; in section it is dull
and opaque, and it is now known to be only partially de¬
composed plagioclase felspar. The pyroxene of Gabbro
is also altered into hornblende, often the bright green
variety called Smaragdite, so that both rocks are then com¬
posed of saussuritic felspar and hornblende ; for such rocks
the name Euphotide is often used. The felspar of a Diorite
may also be altered in the same way. By similar changes,
the pyroxene being partially or wholly altered into horn¬
blende or actinolite, dolerites pass into the condition known
as Epidiorite. If chlorite is developed the rock is called
a Diabase. This change is probably chiefly due to altera¬
tion by water, and some calcite is generally present. A
Diabase is generally greenish, or dull grey.
Altered Basalts.—These are generally known as Mela-
phyres, being dark green or black rocks. The olivine,
when present, is converted into serpentine ; epidote and
calcite arise from the decomposition of the basic sUi-
CHAP. XII.J METAMORPHIC ROCKS.
511
cates, and zeolites are frequently formed in the vesicles and
cavities.
By contact with coal or carbonaceous shale basalt and
dolerite have often been changed into what is called White-
trap, a white, pale green, or pink rock, of which about a
fourth part consists of the carbonates of lime and iron,
with 10 or 11 per cent, of water. This alteration is pro¬
bably due to the extraction of organic acids from the car¬
bonaceous matter and their solution in the heated inter¬
stitial water of the rock, and the reaction of this solution
on the igneous rock.
Altered Andésite.—Andésites which have been altered
by solfataric action contain chloritic minerals developed
from the hornblende and mica, and very often epidote and
iron pyrites. For such rocks Professor Judd has revived
the name of Propylite.
Altered Peridotites.—These form the rock so well known
by the name of Serpentine, of which that of the Lizard, in
Cornwall, forms an excellent type—a compact, massive
rock, mottled with various tints of dull brown, red, and
green, and containing lustrous crystals of a certain variety
of enstatite. Chemically they consist mainly of hydrated
silicate of magnesia and oxides of iron.
B. The Foliated Rocks.
These owe their structure partly to chemical change and
partly to deformation by forcible compression, as will be
shown hereafter, the one action resulting in the formation
of new minerals, and the other in their arrangement along
certain planes in the rock-mass. Sometimes they show a
wrinkled and crumpled structure, which has probably been
produced by extreme pressure acting on a mass in a plastic
or semi-plastic condition. As it is not always possible
to say whether a given foliated rock was originally an
igneous or a sedimentary rock, no attempt is here
made to distinguish between them. Some are certainly
altered Sediments, and some as certainly altered Igneous
rocks.
Schists.—These are the foliated rocks resulting from
512
STRUCTURAX GEOLOGY.
[part ii.
the metamorphism of shales, slates, argillaceous limestones
and fine-grained sandstones on the one hand, or of semi-
crystalline igneous rocks, such as felsites, porphyrites,
andésites, diabases, and melaphyres on the other hand.
Schists are generally micaceous and fine-grained rocks,
splitting easily along the foliation.
Mica-schist is the commonest and most important of
these rocks. It consists of minute fiakes of mica and
small grains of quartz, the latter being generally in less
quantity ; crystalline grains of felspar are often present ;
the mica and quartz are usually arranged in irregular
Fig. 171. Mica-schist from Glen Docherty.
alternating layers, but there is every gradational va¬
riety between a micaceous granulite with shght foliation
and a mylonite of crushed rock, or a corrugated and
crumpled schist. Pig. 171, borrowed by permission from
a paper of Professor Bonney's, illustrates the appear¬
ance of a variety of mica-schist under the microscope.
The unshaded particles are quartz, those with horizontal
shading are aggregations of mica-flakes, and the larger
grains with oblique shading are felspar. The rock of fig.
171 contains a larger amormt of felspar than is usual in
mica-schist. Bed garnets are frequently present, and in
thin sections are generally visible as little " eyes " in the
CHAP. Xll.j
METAMORPHIC ROCKS.
513
foliation. Sericite-schist is a rock with light green, silvery
mica.
Quartz-schist is one in which quartz particles are more
abundant than those of mica. There are, of course, all
varieties, from a quartzose mica-schist, where the propor¬
tions of quartz and mica are nearly equal (fig. 171), to a
rock that is almost a quartzite ; but the name of quartz-
schist is generally applied to rocks which consist mainly
of quartz, with just enough mica to give the rock a
schistose aspect, and to cause it to split along planes of
foliation.
Spotted schist (Fruchtschiefer of the Germans) is a mica¬
ceous schist full of dark-brown or black spots and patches,
most of which are embryo crystals of aluminous silicates,
not sufficiently developed to show regular faces or specific
characters. Kecognizable garnets and cubes of iron
pyrites, however, sometimes occur, and both are common
in all schists. The black spots often consist of particles
of graphite.
Andalusite-schist, Chiastolite, and Staurolite-schists are
those in which certain aluminous silicates are more com¬
pletely developed, so that the mineral species are recogni¬
zable. Both these and spotted-schist occur chiefly in the
metamorphic zones surrounding igneous masses.
Hornblende-schist consists essentially of hornblende,
felspar, and quartz, hornblende generally predominating,
while in some varieties there is little felspar, and in others
very little quartz. The hornblende forms long lenticular
bands alternating with thinner layers of quartz or felspar.
Most hornblende-schists appear to have been derived
from basic igneous rocks.^ "In the field diorites, dolerites,
and aphanites can be seen to pass into hornblende-schist.
Chlorite-schist and Talc-schist are schistose rocks in which
these particular minerals predominate, but many so-called
chlorite-schists are merely green mica-schists. Garnet,
epidote, and magnetite are frequently present in these
schists.
Serpentine-schist is probably in most cases a cleaved
Serpentine.
* See Teall on the "Metamorphosis of Basic Igneous Rocks,"
"Proc. Geol. Assoc.," vol. x. p. 58.
n L
514
STRUCTURAL GEOLOGY.
[part ir.
Calc-schist is a schistose limestone in which accessory
silicates have been developed during metamoi*phism ;
silvery mica is frequent in such altered limestones, and
occasionally tremolite, as at Shinness in Sutherland. Ser-
pentinous marble or ophicalcite is often schistose. Calc-
mica-schist is a variety formed from impure limestones ; the
chief constituents are quartz, calcite, and mica arranged in
foliated layers, the mica being sometimes rendered nearly
opaque by black carbonaceous dust. Dolomite sometimes
occurs in place of calcite.'
Porphyroid and Schalstein are schistose rocks, the one of
an acid, the other of a more basic character, whose origin
is rather obscure, but probably in many cases they are vol¬
canic tuffs altered by pressure and by chemical changes.
In other instances they appear to be compact igneous rocks
which have been similarly altered.
Gneisses.—These are foliated rocks consisting of
quartz, felspar, and mica. They may be coarse or fine in
grain. Recent research tends to the conclusion that most
of them are foliated igneous rocks, and some geologists
doubt whether any true gneiss has originated from a sedi¬
mentary rock. When no shearing from movement under
pressure has taken place the rock is granulitic, when
sheared it is crumpled, and when crushed may be mylonitic.
Eye-structure is often very clearly displayed.
Ordinary gneiss consists of quartz, felspar and biotite,
the felspar being usually orthoclase, arranged in short
lenticular folia, and some of it in large "eyes": the quartz
is often in knots and bands. Garnet is frequently pre¬
sent, while chlorite, epidote, graphite, etc., are common
accessories. Protogine is a variety with altered magnesian
mica, which has been mistaken for talc. Cordierite-gneiss
is a gneiss containing cordierite or dichroite, a silicate of
alumina (with some iron and magnesia), of a clear
blue colour. When hornblende occurs, the rock is called
Hornhlende-gneiss.
If these varieties are compared with those of granite and
tonalité, it will be seen that they closely correspond, and it
may also be observed that when foliation is not strongly
' See Bonney on "Crystalline Rocks of the Alps," "Quart.
Joum Geol. Soc.," 1889, vol. xlv. p. 103.
CHAP. XII.J
M KT AMORPHIC ROCKS.
515
marked it is very difiSeult to distinguish a hand-specimen
of such gneiss from granite. This similarity is, of itself,
sufficient to raise the suspicion that ^eiss has been
formed by the metamorphism and foliation of granitoid
rocks.
Fine-grained gneiss has sometimes been formed from
the crushing in situ of a coarser granitoid gneiss ; compact
gneissic schists which seemed to have been formed in this
manner occur in Koss and Sutherland, and a slice from one
of them near Loch Maree is represented in fig. 172.
With regard to the possibility of some gneiss and granite
Fig. 172. Fine-grained Granulitic Gneiss from Glen Laggan
(Bonney).
having been produced by the extreme metamorphism and
recrystaUization of sedimentary rocks. Professor Bonney
writes :—" It is often asserted that the process of metamor¬
phism has been sometimes carried so far as to obliterate all
trace of the original history of the rock ; in other words,
that rocks exist which, from their relations to other more
intractable, and therefore less altered strata, appear to be
of derivative origin, but which have become absolutely in¬
distinguishable from members of the original or igneous
group of rocks. This passing of a rock from the condition
indicative of metamorphism to that characteristic of its
having solidified from a state of fusion is, in itself, quite
516 STRUCTURAL GEOLOGT. [PAET II.
conceivable, and undoubtedly there are cases where it is
very hard to distinguish, in the present state of our know¬
ledge, between an igneous and a metamorphic rock ; but
it is certainly a fact that the majority of instances
brought forward in proof of this extreme metamorphism
have broken down on careful examination." ^
' From MS. notes by Professor Bonney, who adds, that ' ' since
this was written the advance of knowledge has shown that granite
has, in many cases, been converted into gneiss, while no case of a
franitic rock having been formed out of a mass of schist or gneiss
as been established."
CHAPTEE XIII.
metamoephic rocks,
Considered as Rock-Masses.
IN the preceding chapter the metamorphic rocks have
been described from a lithological or petrological point
of view. We have now to regard them as rock-masses or
components of the earth's crust ; first describing the rela¬
tions which their special structural planes (i.e., cleavage
and foliation) bear to stratification and to one another,
and discussing the evidence of the manner in which these
structures have been produced ; secondly, describing some
actual cases and areas of metamorphism.
I. Cleavage Planes.
Dip and Strike.—The planes of cleavage may be said
to have a dip and a strike just in the same way as planes
of stratification. Their angle of dip varies from the verti¬
cal to as little as 10°, but is more frequently between 50°
and 80°.
Fig. 173 illustrates the varying relations between the
inclination of the strata and that of the cleavage planes.
A represents a section near Llandovery in Wales, where a
series of quartzose grits and sandstones are inclined at an
angle of about 40°, and the cleavage planes dip in the same
direction, but at a much greater angle. In b the cleavage
planes dip in the same direction, while the strata are in¬
clined in the opposite direction. In both cases the strike
of the cleavage planes coincides generally with that of the
518
STRUCTURAL GEOLOGY.
[PAET II.
strata. Occasionally, when the strata have high dips, it
happens that the cleavage planes very nearly coincide with
those of the bedding and lamination.
Their strike is generally constant over considerable areas,
and parallel to the direction of the axes of the principal
flexures. This coincidence between the strike of the
cleavage planes and that of the strata is of itself strong
evidence that the cleavage was caused by the same pressure
as that which caused the plication of the rocks.
One of the best examples of this steady direction in the
strike of cleavage planes is the south of Ireland, over the
whole of which, from Dublin to Mizen Head, the direction
of the cleavage seldom varies much from E.N.E. and
c
a a, Bedding planes, c c, Cleavage planes.
W.S.W., whatever rocks it traverses, and however different
these rocks may be in lithological character and geological
age. The strike of the main axes of flexure is likewise
steady in the same direction. The planes of cleavage are
inclined at very high angles, which generally approach the
perpendicular, but when they have a dip, it is always
to the S.S.E., and this southerly dip appears to be con¬
nected with the special form of the curves into which the
strata have been bent. These are not mere undulations
with vertical axes, but are pushed over in a northerly
direction, so that their axes are oblique, and the strata are
slightly inverted on the northern side, as if the pressure
by which they were produced had acted most strongly
chap. xiii.J metamorphic rocks.
519
from the south ; the dip of the cleavage is therefore
roughly parallel to the dip of these oblique axes, both
being due doubtless to the same cause.
It may, indeed, be stated as a general rule, that when the
axes of the folds and flexures are inclined from the perpen¬
dicular, the average dip of the cleavage planes will be in the
same direction.
North "Wales is another good field for the study of
cleavage. Fig. 174 is a section through the Snowdon
chain, from the Menai Straits in a south-easterly direction ;
the beds, cc c, are conglomerates, the other beds being
parallel to them, and the fine strise are cleavage planes
striking with the beds to the N.E., but cutting across them
in the direction of the dip ; for while the beds undulate at
The fine lines indicate cleavage planes, but should have been drawn
parallel to the an'ows which show the slight variations in their
dip.
various angles, the cleavage dips slightly on either side of
the vertical, viz., N.W. at 80° or 85° from a to b ; S.E. at
80° to 85° from b to c ; and 80° to the N.W. from c to d.
From these facts it may be inferred that the rocks were
folded first and cleaved afterwards, though possibly not
long afterwards ; and that some slight movements had
taken place after the cleavage was set up.
In North Wales the prevailing strike of the cleavage is
N.E. and S.W., because this is also the direction of the
main lines of flexure ; but where the strata are bent round
so as to have a different strike, the strike of the cleavage is
altered in a similar manner, so as to coincide with that of
the beds.
" In the Berwyn chain, where the beds curve regularly
round, from a N.E. and S.W. strike along the Bala and
520
STRUCTURAL GEOLOGY.
[part II.
Corwen valley, to an E. and W. strike along the vale of
Llangollen, the strike of the cleavage follows with equal
regularity, the cleavage plains dipping W. 20° N. at 30° in
the country between Bala and Llangynnog, curving round
as they approach Corwen, and striking nearly due E. and
W. on both sides of the Dee, between Corwen and Llan¬
gollen, with a dip almost invariably to the north at a high
angle." ^
Cases have been noticed where the dip of the cleavage
appears to have some connection with the dip of the beds.
Thus Darwin has described instances in South America
where the cleavage planes dip inward on either side of
large anticlinal curves, so as to exhibit a fan-like arrange¬
ment ; but such exceptions to the general rule are probably
explicable on the supposition that the dip of the cleavage
has been affected by subsequent movements, the cleavage
being antecedent in point of time to the production of these
particular sets of anticlinals. It is quite possible that a
district may at one period of time be subjected to very
great compression, capable of producing both contortion
and cleavage ; and that after a period of quiescence, during
which the beds acquired fixed positions, renewed compres¬
sion may give rise to another set of curves, or even to a
second set of cleavage planes. The range of the Andes is
exactly one of the districts where we should expect to find
evidences of repeated periods of compression, because there
is other evidence that its elevation has been the result of
repeated movements (see p. 70). Cases in which a second
set of cleavage planes have been noticed, with a different
strike or dip to the first set, are mentioned by Sedgwick,
Phillips, and others.
Influence of Rock-texture on Cleavage.—Cleavage
is always most perfect in the finest grained rocks, splitting
them into an indefinite number of thin plates. The
coarser the rock, the fainter, the wider apart, and the
more rough and irregular do the cleavage planes become ;
and in the case of thick-bedded sandstones cleavage gene¬
rally divides the rock into coarse slabs only, the upper and
under surfaces of the beds often breaking into dog-toothed
' Jukes' " Manual of Geology," second edition, p. 267.
chap. xiii.] metamorphic rocks.
521
indentations, and thus indicating the compression they
have undergone. It sometimes happens that the direction
or dip of the cleavage planes is temporarily altered in cer¬
tain beds, where it traverses a series of beds of differing
hardness and texture.
Pig. 175 is from a section at Killeen, near Cork, in the
Old Eed Sandstone. The beds, a a, contain about 80 per
cent, of silica, with only small quantities of alumina and
iron ; these show no signs of cleavage. The beds, b b, are
impure sandstones, containing only 68 per cent, of silica,
with 25 per cent, of alumina and iron ; these are distinctly
cleaved, and split vertically into flags along the cleavage
planes. The beds, o c, are intermediate in their characters,
containing 76 per cent, of silica, with about 20 per cent, of
alumina and iron ; and in these the cleavage is not only
more imperfect, but has a strong dip to the south.' Pro¬
fessors Harkness and Blyth mention other instances in the
island of Valentía, where the dip of the planes changes
several times in passing from one bed to another, when
these are of different mineral composition.
Proofs of Compression.—It has been stated (p. 505)
that cleavage is a structure produced by intense compres¬
sion, but it is desirable to furnish the student with proofs
of this statement, as derived from observations in the
' Harkness and Blyth, "Trans. Edin. Geol. Soc.," 1855.
522
STRUCTURAL GEOLOGY.
[PAET II.
study and in the field. By microscopical examination Mr.
Sorby found that the minute particles of slate lie more
flatly in the cleavage planes than in any other direction ;
whence he inferred that they had been lengthened in the
direction of the cleavage planes, or else, if they were
originally of unequal dimensions, they have been re¬
arranged so that the longer axes coincide with the planes
of cleavage. Mr. W. M. Hutchings says : " The base and
main constituent of all these slates (from Wales and Corn¬
wall) is a small grained mica, mostly lying flat in the
planes of cleavage of the rock." ' In the case of coarse¬
grained rocks this rearrangement of the component parti¬
cles may be easily observed ; thus. Sir A. Kamsay describes
a conglomerate near Llyn Padarn in Caernarvonshire as
consisting of slaty pebbles in a slaty matrix, the whole
being affected by cleavage remarkable on account of the
pebbles being rearranged with their longer axes parallel to
the cleavage planes, and looking as if they had been
elongated or pulled out in that direction.^
Professor G. Cole ' describes the microscopic structure of
slate in the following terms:—"All the microlites and
grains seem lying with their longer axes parallel with one
another. . . . The transparent constituents can sometimes
be identified as mica. . . . The black amorphous clay
particles or groups of kaolin flakes are pressed out into
extremely flattened lenticles, so that when cut perpendicu¬
larly to the cleavage surfaces fine dark lines run parallel
and close-set through the slide."
The same appearances are exhibited by the fossils which
occur in slate rocks ; their dimensions are always changed,
so that the fossil is distorted, being lengthened in one
direction, and shortened in another. Thin shells such as
Lingulœ and the tests of Trilohites are especially affected,
the latter being sometimes minutely wrinkled in lines
parallel to the cleavage.
It may also be observed that when harder beds, such as
bands of sandstone or limestone, occur among slates, they
' See his papers on the " Probable Origin of some Slates," " Geol,
Mag.," 1890, pp. 264 and 316.
^ " Geology of North Wales," " Mem. Geol. Survey," p. 145.
' " Aids in Practical Geology," Grilfin and Co., 1891.
CHAP. XIII.J METAMORPHIC ROCKS.
523
are strongly contorted in the manner represented in fig. 176,
their dimensions being contracted at the sides, and ex¬
panded at the tops and bottoms of the curves, while the
beds of slate above and below are little, if at all bent.
This seems to show that the material of the slates has been
compressed at right angles to the cleavage planes, and
lengthened along them so as to allow of their being
squeezed into the same contracted space as the sandstones,
without much bending of the surfaces of the beds. Mea-
Fig. 176. Contorted Beds in Slate near Ilfracombe (after Sorby).
surements along some of these contorted bands show that
the whole mass has been compressed into one-third of the
linear or horizontal space which it originally occupied.
Mr. Sorby remarks " that these and numerous other cases
in North Devon are analogous to what would occur if a
strip of paper were included in a mass of some soft, plastic
material which would readily change its dimensions. If
the whole were then compressed in the direction of the
length of the strip of paper, it would be bent and puckered
624
STRUCTUKAL GEOLOGY.
[part ii.
up into contortions, while the plastic material would change
its dimensions without undergoing such contortions ; and
the difference in distance of the ends of the paper, as mea¬
sured in a direct line, or along it, would indicate the change
in the plastic material."
Conditions necessary to produce Cleavage.—The
origin of cleavage has recently been discussed by the Eev.
O. Fisher, who thinks that the phenomena indicate some¬
thing more than mere compression and packing of the rock-
particles, and believes that a movement of the nature of a
shear, causing an actual elongation of these particles, is
necessary for the production, of cleavage.
A shear is the result of a stress or pressure so applied
as to make one portion of a body move over another portion.
When the form of a body under stress is permanently
altered without solution of continuity, it is said to be
plastic, and the movement of a plastic body under sheaiing
stress may be called a viscous shear. When the stress is so
great as to overcome the friction of the component parti¬
cles the body shears as a sohd, that is to say, it gives way
along certain planes and breaks into slices, each of which
moves over that beneath it. The planes thus formed are
called shear surfaces, and the thrust faults described on
p. 454 are surfaces of this kind.
In his earlier papers Mr. Fisher assumed that cleavage
planes were also shear surfaces,' but subsequently per¬
ceived that they could not be such surfaces, and now
regards them as resulting from the elongation of the com¬
ponent particles by a viscous shear." He describes a case
in which the cleavage of a slate is crossed obliquely by
what appear to be planes of shear, and he shows that if a
circular area were sheared between two of these planes it
would be distorted into an ellipse, the longer axis of which
would be nearly parallel to the cleavage. Hence he con¬
cludes that it was the elongation of the rock-particles by
the shear which gave the rock its cleavage. In Mr. Fisher's
own words, "it is due to the deformation of the consti¬
tuents of a (comparatively) homogeneous and ductile rock,
' " Geol. Mag.," Dec. .3, vol. i. p. 271.
Op. rit., p. 398, and "Physics of the Earth's Crust," second
edition, p. 269.
CHAP. XIII.J METAMORPHIC ROCKS.
525
and it follows the flattened faces of the deformed particles."
[Op. cit., p. 370.)
Mr. Fisher has been kind enough to explain by letter
more fully what he means by " deformed particles," and it
seems he means small aggregations of particles, or what
may be termed molecules; but as the existence of such
molecules in slates is very doubtful, and as it is certaiû that
the individual mica-flakes lie in the planes of cleavage, the
author cannot accept Mr. Fisher's view, but believes that
cleavage is a result of the rearrangement of the component
particles of a rock. At the same time, he admits that a
mass of clay or shale compelled to change its dimensions
will shear, and that the resulting movement may be such
as will satisfactorily explain the distortion of fossils, peb¬
bles, or nodules contained in the mass.
It must be remembered, however, that a shearing stress
may cause more than one kind of movement within the
mass, and that the particular movement considered by
Mr. Fisher may be only the final movement developed by
the continued pressure. The pressure, or the stresses set
up by it, act more or less horizontally, and cause compres¬
sion combined with upward expansion ; and it is quite
conceivable that the first result of such compression of a
plastic mass consisting chiefly of minute flattish particles
would be to cause a kind of irregular upward flow, during
which the particles, by mutual friction among themselves,
without elongation or deformation, would be simply re¬
arranged with their broad sides facing the pressure and
their longer axes in the direction of least resistance. This
seems to be the view held by Sorby, Tyndall, and others,
who actually produced cleavage in various substances by
subjecting them to compression, only they did not call the
motion of the component particles a shear.
But a cleavage so produced wiU not entirely account for
the distortion of fossils above mentioned : here Mr. Fisher
steps in, and shows that if the final result of the shear is
to cause the component particles to move along parallel
planes, any spheroidal area in the compressed mass will
necessarily be deformed into an ellipsoid ; this will ap¬
parently take place by a slipping of the particles past
one another without disturbance of their parallel positions.
526
STRUCTURAL GEOLOGY.
[PAET II.
"but perhaps with a tendency to bend or incline them in the
direction of the shear; that is to say, the shear forces them
to take up a position which is represented by the direction
of the longer axis of the ellipsoid of distortion. Such a
movement is just what is likely to happen when a force is
overcoming the resistance offered by the cohesion of the
particles, and when carried a degree further it will result
in the development of shear surfaces such as appear in the
case referred to by Mr. Fisher.
There are also some cases where the cleavage planes do
coincide with shear surfaces ; the Ausweichungs-clivage of
the Germans, for which Prof. Bonney has proposed the
name " strain-slip cleavage."
If the view just expressed is correct, there must be cases
of cleaved rocks without distortion of fossils, such bodies
only showing elongation parallel to the planes of cleavage.
It is believed that such cases do exist, but this point
requires further investigation.
It has been observed (1) that in many cases the pressure
has come from one definite direction, and (2) that where
large masses of very hard rock occur in a cleaved area the
cleavage is always most strongly developed on the side
which faced the pressure, and (3) that in such localities
shear suiiaces are often observable, but that they have a
steep inclination and cut obliquely across the cleavage ;
these are facts which seem to indicate the foUovring as
being the conditions necessary for the production of
cleavage :—
A resistant mass, or several such masses, of hard rock,
which serve as relatively fixed abutments, a lateral force
producing compression of the softer and more plastic
rocks against these abutments, and causing an upward ex¬
tension of the plastic beds, accompanied by such a re¬
arrangement of their component particles that the flat flakes
of mica and kaolin come to lie in parallel planes.
Continued application of pressure to such a mass may
develop a shearing stress, the motion taking the direction
of least resistance, which under such conditions will be
obliquely upward, pulling out the rock-mass in that direc¬
tion, and ultimately overcoming the friction and producing
shear surfaces. The difference between this shear and that
CHAP. XIII.J
MET AMORPHIC ROCKS.
527
producing a horizontal thrust plane or shear fault, is that
in the one case the force was not suflBcient to over¬
come the resistance of the abutment, in the other it was.
Hence the difference in the hade of the shear planes, the
one hading at a high angle, the other at a low angle.
This difference may be seen also in the section, fig. 178,
where the reverse faults with steep hades have probably
been formed before the resistance of the abutment was
overcome, while the thrust planes were formed after this
abutment gave way. In the N.W. of Scotland the abut¬
ment was evidently the gneissic mass of the Hebrides.
We may conclude, therefore, that a region undergoing
compression will first have its component strata flexed and
folded; continued or subsequently renewed pressure will
cause, first cleavage, then reversed faulting, and finally
more or less horizontal over-thrusts.
Foliation.
Until recently, writers on foliation were much occupied
with the question whether foliation coincided with cleavage
or with stratification. Sedgwick and Darwin were of opinion
that foliation was almost always coincident with cleavage,
and that it was, in fact, only a further development of
the same process. Sedgwick, however, then believed that
cleavage itself was produced by the action of crystalline
and polar forces, instead of by pressure.
Jukes maintained that the coincidence of foliation with
cleavage was only an accidental one, and that it was only
likely to occur in cases where a considerable thickness of
argillaceous deposits had been converted into a tolerably
homogeneous mass of slate before it was subjected to
the further metamorphic processes which produced folia¬
tion.
Lastly, it was the unanimous testimony of observers
in the metamorphic areas of Ireland and Scotland that in
those countries the foliation appeared to coincide with the
original stratification, and they appealed to cases where
quartzites and limestones were interstratified with schists,
the foliation of the latter coinciding with the dip and
strike of the former.
528
STRUCTURAL GEOLOGY.
[PAET II,
It was therefore the general conclusion that foliation
coincided either with cleavage or with stratification, and
more often with the latter ; and it was inferred that the
arrangement of the crystalline minerals in folia was regu¬
lated by the previously-existing lines of separation.
The fact is, the structure called foliation has not always
been produced in the same way, and there are probably at
least three different processes by which this structure may
be set up.
Eecent researches, especially those of Lapworth and
Callaway, and more lately by the oflicers of the Geological
Survey, have demonstrated that in regions like the north
of Scotland the main planes of schistosity or fobation are
those of shear under intense lateral pressure, and that the
foliation is developed with the same direction through
stratified and unstratified rocks alike. In the words of
Professor Lapworth,' " the planes of schistosity are those
along which the rocks yielded to the lateral pressure. . . .
These planes cut the rock up into lenticular patches or
phacoids, which, like the yielding planes themselves, are
of all gradations of size, from the mountain masses riding
out along the great overfolds and overfaults down to the
"eyes" of augen-schists. The mechanical effects wrought
by the shearing and deformation of the phacoids show of
necessity a corresponding gradation, the conditions at one
extreme giving rise to coarse rock-breccias; in medium
cases to flaser structures and foliation ; at the other extreme
to compact stringy mylonite, in which the original material
has been torn and ground to rock-dust. Parallel with these
mechanical changes, but not necessarily accompanying them,
we note a series of chemical changes of rising grades of
importance, a larger and larger portion of the rock which
is undergoing deformation becoming recrystallized, until
finally it may all be transformed into fobated crystalbne
rock."
The varieties of the schists produced in this way are due
partly to the relative degrees of alteration, and partly to
the original differences in the structure and composition
of the rocks acted upon; for what are now schists may
' In Page's " Introd. Text-book of Geology," twelfth edition,
p. 113.
CHAP. XIII.J METAMORPHIC ROCKS.
529
have been originally either sedimentary or igneous rocks,
or even gneisses and schists formed by a previous meta-
morphism. "In fine," to quote Professor Lapworth again,
"a metamorphic area is a petrological complex whose
altered rocks have a common foliation. The stnke of
foliation in such a district is related to the general strike
of the sheets of sedimentary or crystalline rocks which
formed its main masses at the time of its final folding and
shearing: the dip of the foliation is simply the local direc¬
tion of shear, i.e., is related to the direction in which the
mass was giving way." '
Professor Bonney, however, maintains that this is not
the only way in which foliation is produced. He fully ad¬
mits that pressure has been the chief agent in producing
the more prominent schistose structures of the north-west
Highlands, but thinks that foliation often exists where
there is no cleavage and no shearing, and where the
mineral banding has clearly been guided by the stratifica¬
tion of the materials. He mentions districts where both
kinds of foliation are distinguishable in the same mass of
rock, the stratification-foliation being always the first-
formed and generally dominant, though sometimes more
or less obliterated by subsequent pressure-foliation and
shearing structures. In the Swiss Alps this secondary
foliation, due directly to pressure, " may be traced in every
degree, from the stage where the sheen surfaces are quite
subordinate to the stratification-foliation, to that in which
the latter has been wholly obliterated by cleavage-foliation,
when the rock splits up into thin slabs, sometimes almost
films of a friable micaceous schist which we might almost
call a slate." ®
There is, moreover, another way in which foliation may
be produced, to which attention has recently been directed
by General Macmahon and Professor Bonney, in connection
with the foliated gabbro of the Lizard district. This rock
' I have given Professor Lapworth's description of pressure nieta-
morphism, not only because it is tersely and clearly expressed, but
because be was the first to publish the deformation theory in
its complete form, and to explain the structure of the Scottish
Highlands.
" " Pres. Address tc Geol. Soc.," 1886, p. 69.
M M
530
STRUCTURAL GEOLOGY,
[PAET II.
there occurs in dykes which break through serpentine and
schist, and some parts of these dykes are distinctly foliated ;
the foliation is generally parallel to the walls of the dykes,
often occurring only near the walls, but sometimes trending
inwards obliquely and blending into the unfoliated portion
of the mass. The folia are in some cases so thin and small
that the rock resembles a schist, but in other parts the
grain is coarse and "eye" structure is not uncommon.^
The authors above mentioned regard this foliation as a
kind of fluxion structure produced by movements in the
mass when it consisted of crystals floating in a viscid
magma, and when its outside portions were on the point
of solidification.
Banded Gneiss.—There is still another structure exhibited
by certain rock-masses which is somewhat analogous to
fluxion-foliation, but is on a larger scale. In these rocks
distinct layers or bands of different rock-material alternate
vdth one another in such a manner as to simulate strati¬
fication. Thus the banded gneiss of Cornwall consists of a
dark dioritic rock veined or banded with a light-coloured
rock which resembles a finergrained granite or eurite.
There is no evidence of intense pressure or crushing, and
it is supposed that this banding has been brought about
by the injection of an acid magma into a basic magma or
rock, the former having a high temperature and so far
softening the latter that it was pulled out and made to
flow with the more liquid material; the two kinds of rock-
çtuff being thus welded together without being completely
mixed before the final crystallization took place.^
The folia of schists are frequently wrinkled and corrugated
in a remarkable manner, giving the rock a peculiar crumpled
and gnarled appearance. Sir A. Ramsay has suggested that
these very lines are those of a previously induced cleavage
which was only partially obliterated by the foliation, the
planes of cleavage being bent and crumpled by farther
' "Quart. Journ. Geol. Soc.," vol. xlvii. p. 487.
^ Bonney and Macraahon, " Quart. Journ. Geol. Soc.," vol. xh-ii.
p. 477. See also a paper by Messrs. Hill and Bonney on the Schists
and Gneisses of Sark, read before Geol. Soc., Jan. 20th, 1892, in
which they describe the genesis of a banded biotite-gneiss out of a
horn blende rock invaded by a fine-grained granite.
CHAP. XIII.j METAMORPHIC ROCKS.
531
pressure when the mass was in a plastic condition. This
explanation assumes the existence of a previous cleavage,
and it is possible that in many cases the corrugation is
simply due to subsequent compression acting on the folia
of a schist, and that it is therefore analogous to the crum¬
pling of the original laminae of a slate. It may be the
effect of pressure acting on the mass after the layers or
folia had been formed.^
2. Areas of Metamorphism.
The reader may already have gathered from statements
in the preceding pages that the metamorphism of rocks
has not been accomplished by one set of agencies only, or
Under one set of physical conditions. As a matter of fact,
the rocks which are usually grouped together as metamor-
phic have resulted from the operation of several distinct
physical processes ; one being the intrusion of large
masses of igneous rock, another being the development
of stresses and strains in a portion of the earth's crust.
While as to the manner in which stratification-foliation
has been produced over large areas we must at present be
content to confess our ignorance. It is possible that in
time the microscope will disclose the existence of radical
differences between rocks formed in such different ways,
and it is already certain that some varieties of rock are
only formed under certain conditions. It is therefore
desirable to distinguish clearly between the different kinds
of metamorphism : the first has been called Local or
Contact Metamorphism, the second Fressure Metamorphism,
the third Regional Metamorphism.
Local or Contact Metamorphism.—Some of the
metamorphic effects produced by the intrusion of igneous
rocks have already been mentioned. The mutual alteration
resulting from the contact of granite veins with limestone
was mentioned on p. 485 ; again, the manner in which a
mass of slate may be altered into mica-schist by the
. ' This passage remains as printed in the first edition ; the view
expressed has been confinned by recent observations in Scotland.
See " Quart. Journ. Geol. Soc.," 1888, p. 378.
532
STRUCTURAL GEOLOGY.
[part hi
intrusion of granite bosses was illustrated in fig. 481.
But in order to understand the nature of the process by
which such effects are produced, it is requisite that these
and other similar instances should be more closely and
minutely examined.
Alteration by ByJces.—1. A simple instance of local meta-
morphism is presented by the chalk in contact with the
basaltic dykes of Antrim. By the intrusion of these dykes
the chalk has been converted into a crystalline stone ; the
amount of alteration being greatest near the basalt, and
gradually decreasing outwards, but sometimes extending
to a distance of 8 or 10 feet from the wall of the dyke.
"The extreme effect," says Dr. Berger, "presents a dark
brown crystalline limestone, the crystals running in flakes
as large as coarse primitive (i.e. metamorphic) limestone ;
m d d m d m
Fig. 177. Alteration of Chalk by Basalt Dykes, Antrim.
d d, Dykes, m m. Altered portions of the chalk.
the next state is saccharine, (gradually becoming) fine¬
grained ; a compact variety having a porcellanous aspect,
and a bluish grey colour succeeds ; this towards the outer
edge becomes yellowish white, and insensibly graduates
into the unaltered chalk."
The annexed sketch, fig. 177, represents the plan of three
dykes as seen on the beach of Rathlin Island, Antrim, which
have altered the contiguous portions of the chalk, m m,
into the granular marbles above described.
2. Another instance of the powerful effect of a dyke has
been described by Professor Henslow as observed near Plas-
Newydd in Anglesea. In this case a broad dyke of dolerite
traverses strata of shale and argillaceous limestone; the
metamorphic effect begins to be perceptible at a distance of
about 30 feet, the shale becoming compact and mdurated.
CHAP. XIII.J METAMORPHIC ROCKS.
633
and losing its fissile character, though the lines of lamina¬
tion are still visible ; near the dolerite it is converted into
a hard jaspery porcellanite, while crystals of garnet and
analcime are developed more or less throughout the altered
portion of the shale. The limestone undergoes similar
alteration, becoming gradually compact and crystalline in
approaching the trap.
It often happens, too, that the igneous rock itself is
likewise altered near the contact surfaces, especially if it is
intruded into rocks of a calcareous or carbonaceous nature ;
basalt often loses its dark colour, assumes tints of pink
and green, or even becomes quite white, and chemical
analysis shows that such white traps contain from 9 to 10
per cent, of carbonic acid, and 10 or 11 per cent, of water.
Excellent instances of such mutual metamorphism occur
in Fifeshire, which has been the scene of extensive and
repeated volcanic intrusions. The results of one of these
intrusions may be studied on the shore at Eoss End near
Burntisland, where the calciferous sandstones are invaded
by a sheet of dolerite. The sandstones are converted into
quartzite, and all stages of alteration can be traced in the
lower part of the dolerite, passing from dark green, through
light green, to pink and white, and in one place it is bright
fed, with amygdaloidal inclusions. The main sheet sends
off intrusive branches and veins of white trap into the
quartzite, fragments of which have been broken off and
included in the igneous rock.
Similar instances are found in the South Staffordshire
coal-field, as described by Mr. Jukes,^ who specially men¬
tions one in the Grace Mary colliery, where a part of the
Tenyard coal and associated sandstone is penetrated by an
irregular sheet of white earthy trap, which has probably
proceeded from the black basalt of the neighbouring
Eowley Hills. The sandstone has not been much altered
by the contact, but the coal has lost its bright lustre and
much of its inflammable character.
Alteration hy Granitic Masses.—1. As an instance where
the effects of intrusive bosses of igneous rock can be easily
examined we may take those of Cornwall. In this county
' "Geology of S. Staffordshire Coal-field, Mem. Geol. Survey."
534
SXEl'CTURAL GEOLOGY'.
[part ii.
four bosses of granite, which are probably parts of one
continuous underlying mass, are intruded into the clay
slates which are locally termed killas. The metamorphic
zone surrounding any of these bosses varies from a quarter
to half a mile in width ; within these limits the killas is
converted into micaceous schist, and in the immediate
neighbourhood of the granite into a still harder semi-
crystalline and compact felspathic rock (hornfels).
2. The metamorphism of the Silurian slates by the
granite of Leinster, already described (pp. 481 and 482),
supplies an instance in which a still wider zone of rock has
been altered in a similar way. Mr. Jukes has given the
following description of this district : '—" The great mass
of granite which has been intruded into the clay slates of
Leinster forms a continuous range of granite hills from
Dublin Bay to the neighbourhood of New Ross, a distance
of 70 miles. Between this range and the coast other
smaller intrusive bosses make their appearance at the
surface. The unaltered clay slates exhibit various tints
of dull grey, green, blue, purple, and black ; they are
always without lustre, and generally of a dull earthy
texture. Small bands of grey siliceous grit often occur in
them.
" Wherever the granite comes to the surface a belt of
slates surrounding it is converted into mica-schist, with,
in some few places, beds that might be called gneiss.
Crystals of garnet, schorl, andalusite, staurolite, etc., make
their appearance in these altered slates in greater and
greater abundance as they approach the granite. The
width of the metamorphosed belt is generally proportioned
to the size of the granite mass which it surrounds. Round
the smaller granite bosses it is sometimes not more than
50 yards wide ; round the main granite mass it sometimes
reaches to two miles. It matters not through what part
of the slate rocks the granite rises, or which beds strike
towards the granite,—they are all found to be affected in
the same way as they approach it.
" In going towards the main granite ridge it is found
sometimes at a distance of two miles from the outcrop of
' " Students' Manual of Geology,'" second edition, p. 275.
CHAP. XIII. J METAMORPHIC ROCKS.
535
the granite (which, is, however, much nearer probably in a
vertical direction), that the slates have acquired a ' glaze,'
as it were, or micaceous lustre, with a soapy feel. This
micaceous appearance increases as we approach the granite,
till at last distinct plates and folia of mica are to be seen,
and the whole assumes the ordinary character of mica-
schist, occasionally passing into a kind of gneiss. Some
of these gneissic layers are obviously beds of sandstone
originally interstratified with the shales, the rocks having
all the appearance of interstratified shale and sandstone at
a distance, and until they are broken open, and are found
to have the aspect of mica-schist and gneiss." '
By French writers the term aureole has been introduced
to designate the concentric zone of metamorphosed rocks
which surrounds an intrusive mass of igneous rock. Thus,
in describing the phenomena of contact metamorphism
round the granitic bosses of Asturia in Spain, Dr. Barrois
says : " The first metamorphic effect is simply a change of
structure, consisting of the development of a crumpled or
goffered structure, without any new combinations being
formed among the elements of the rock. Nearer the granite
the metamorphic effect is different and more intense ; the
minerals whose substance is disseminated in a pulverized
state within the rock have crystallized in consequence of
molecular changes, the particles of a similar nature being
attracted to one another," and forming crystals of mica,
chiastoUte, etc.^ These chiastolite shales become more and
more micaceous as one approaches the granite, until they
become regular foUated schist. Three distinct concentric
zones can, therefore, be recognized in the aureoles round the
granites of Asturia, and may be distinguished as follows,
commencing with the outermost:—
1. Zone des schistes gaufrés (wavy schists).
2. Zone des schistes maclifères (spotted schists).
3. Zone desLeptynolithes (gneissic and quartzose schists).
^ The reader will see that Jukes is only describing the general
aspect of the rocks, which do not really exhibit the microscopical
structure of mica-schist and gneiss.
' " Recherches sur les terrains anciens des Asturies et de la
Galicie, Mem. Soc. Geol. du Nord," tome ii. 1882, p. 92.
536
STRUCTURAL GEOLOGY.
[part ii.
Dr. Barrois is careful, however, to point out that this suc¬
cession of aureoles is only true in a general sense, and that
layers of micaceous schist with crystals of chiastolite are
often intercalated among the less highly metamorphosed
schists. Such intercalations are doubtless dependent upon
differences in original mineral composition, the crystals
being more easily developed in some beds than in others.
Describing the effects of the granitic intrusions of the
Vosges, Professor Eosenbusch also distinguishes three
zones which he terms :—
1. Zones of spotted clay-slate (knoten thon-schiefer).
2. Zones of spotted mica-slate (knoten ghmmer-schiefer).
3. Zones of massive hornstone (hornfels).
The homstone of the innermost zone is a hard, compact
rock, sometimes of a jaspery aspect, sometimes more crystal¬
line, and generally containing crystals of andalusite. The
microscope showed it to be really crystalline and to consist
of quartz, two kinds of mica, andalusite, magnetite and
haematite.
The effect of granite on a mass of sandstone has been
well described by Dr. Barrois in connection with the granite
of Guéméné, in Brittany. The unaltered sandstone consists
of quartz and white mica with some fen-uginous matter.
In the first zone the minerals are the same, but both
are recrystalhzed, and the rock is a quartzite ; the mica
changed to a black variety, probably by absorption of
iron. Nearer the granite sillimanite (a variety of andalu¬
site) in acicular crystals and magnetite appear. Close to
the granite the rock is traversed by granitic veins of all
sizes, and is so impregnated with granitic material that it
becomes a felspathic quartzite.
District and Regional Metamorphism.—The cases
of alteration above described demonstrate that the intrusion
of igneous rock into stratified rock is sufiBcient to convert
portions of the latter into metamorphic rocks by a recrys-
tallization of the rock material. Moreover, since the ex¬
tent of this metamorphism appears to be roughly propor¬
tionate to the mass of the intrusive rock, it might be
supposed that the cases of metamorphism on a larger scale
were principally due to the proximity of still larger masses
CHAP. XIII.]
METAMORPHIC ROCKS.
537
of intrusive igneous rocks. This, however, would le a
false inference, for we could not examine many metamor-
phic districts without finding that the extent of metamor-
phism was out of all proportion to the mass of purely in¬
trusive rock in the same area. The Scottish Highlands, for
instance, consist mainly of metaniorphic rocks, yet they do
not exhibit any mass of intrusive granite so large as
that of Leinster, the alterative effect of which only
extends to a maximum distance of two miles. Again, in
the Swiss Alps, where the whole mountain masses consist
of highly metamorphosed strata, intrusions of granite or
other plutonic rock are of rare occurrence, and where they
do exist cannot be connected with metamorphic action of
any importance.
It is clear, therefore, that the mere contact with deep-
seated igneous rock is not sufiäcient to produce wide areas
of metamorphism ; but the same study of metamorphic
districts which would lead us to discard this hypothesis
would elicit the fact that in some districts metamorphic
action has always been accompanied by great contortion
and plication of the strata with overfolds and overthrusts,
and that the amount of this plication and dislocation does
appear to have a direct relation with the intensity of the
metamorphism.
In such districts everything points to the combined
infiuence of intense pressure and heat, with a resulting
movement of elevation. Mr. Mallet has also suggested
that the energy of the force employed in crushing and
contorting the rocks may be transformed into heat, and
that the amount of heat evolved during this process might
be sufiBcient to soften and partially fuse the rocks which
are thus affected. But-geologists have not found any evi¬
dence in nature which tends to support this hypothesis.
Moreover, Professor Bonney has pointed out that in some
metamorphic areas there is very little evidence of pressure,
and that in others, e.g., the Alps, there is clear evidence
that the pressure did not act till after the rocks had become
crystalline. Again, in Scotland there are large areas
where the metamorphism is not due to lateral pressure or
crushing, though there are certain districts where special
kinds of schistose rock have been produced by pressure
538
structural geology,
[part ir.
out of previously foliated rocks. Professor Bonney con¬
sequently maintains that it is a mistake to include these
special cases under the head of regional metamorphism ; this
term should be reserved for the larger areas of metamor-
phic and crystalline rocks, such as the Scottish Highlands
as a whole, the crystalline region of the Alps, and large
parts of Norway, Greenland, and Canada. The phenomena
of the smaller districts, often included in these tracts,
which have been modified by pressure may be described
under the head of district metamorphism.
As an instance of such a district we may take the N.W.
Highlands, the phenomena of which have been specially
studied by Professors Lapworth and Bonney, Dr. Callaway,
Messrs. Peach, Home, and other ofiicers of the Geological
Survey. Three great formations enter into the structure
of this region; the lowest and oldest is the basal gneiss of
Sutherland and Ross ; the second is a massive formation
of red sandstone and conglomerate ; the third is a series
of grits, quartzites, shales and limestones. Before the
production of the dislocations and deformations to which
the present arrangement of the rock-masses is due, the
basal gneiss formed a continuous platform supporting the
two other formations ; the red sandstones, which are in
places more than 4,000 feet thick, thinned out eastward
and allowed the uppermost stratified series to rest directly
on the gneiss ; the general strike of this series being from
N.N.E. to S.S.W., and their general dip being to the
E.S.E.
In the western part of the region the original relations
of the three formations are clearly visible, and the rocks
can be recognized as those above mentioned; it is seen
also that the basal gneiss received its foliation before the
accumulation of the sandstones. In the central and eastern
districts, however, the whole mass has been plicated and
dislocated, cut up by reversed faults, and sliced by thrust
planes, and the severed blocks and slices have been piled
on to one another in a manner that can only be realized
by actual inspection of the country, or of the detailed
sections drawn by the geological surveyors.' At the same
' See Report of recent work in the North-west Highlands of
Scotland, "Quart. Journ. Geol. Soc.,'' vol. xliv. p. 378.
East.
f a ii e a a a m
Eig. 178. Diagraniatic Section across the North of Scotland.
W. Dnrncss Basin. Eriboll Area. E.
Fig. 179. Generalized Section across the Durness-Eriholl Kegion of Sutherland
(after Messrs. Peach and Home).
w. Siliceous Schists. d. Calcareous Grits and Mudstones.
/, Frilled Schists. Quartzite Group.
e, Limestone Series. h, Torridon Sandstone.
a, Archfean Gneiss.
It will be noticed that the faults in the western half of the section are of the normal kind, hading to the
downthrow, while tjiose in the eastern half arg all reversed overthrust faults or thrust-plançs,
640
stltuctural geology.
[part ii.
time, in consequence of these mechanical movements and
of the chemical action thereby set up, the rocks have all
been converted into varieties of gneiss and schist, and the
foliation of the previously existing gneiss has been more
or less obliterated by the secondary foliation which it has
ac(|uired in common with the other rocks. The igneous
rocks too, which occur as intrusive masses and dykes in
the stratified series, are in the central district converted
into augen-schists,hornblende, and sericite-schists. Fig. 178
is an attempt to give a general idea of the structure of this
region, but on such a small scale it is impossible to convey
more than a feeble outline. Fig. 179 is a portion of it on a
larger scale ; and the reader should also refer to fig. 144,
p. 454, to realize the kind of faulting.
The following extracts from the Export of the Geological
Surveyors describe the extent to which the metamorphism
of some of the rocks has been carried.
First, as to the gneiss, they note a gradual increase in
the alteration of this rock, and in the obbteration of the
first foliation as it is followed from west to east. De¬
scribing the belt of sheared gneiss and green schist which
underlies the Moine thrust plane, they say ; " The Archaean
gneiss has been rolled out into a finely laminated slate, or
slaty schist (mylonite), breaking into thin folia like leaves
of paper. All the various stages of deformation, from the
crushed Archaean gneiss on the one hand, to the laminated
slate on the other, can be clearly traced. The original
constituents of the gneiss have been comminuted, but here
and there broken fragments of the felspars occur, which
are invariably drawn out in the direction of movement. . . .
These finely laminated schists or slates show beautiful
examples of fluxion structure, and their foliation-surfaces
display closely set lines or ' striping,' indicating the direc¬
tion of movement of the particles over each other, the
general trend of the latter being E.S.E. Associated with
these slates are certain belts of ' frilled ' dark green schists,
a detailed study of which points to the conclusion that
they have been formed mainly out of dark hornblendic
gneiss, the folia having been piled on each other by minute
major and minor thrusts." {Op. cit., p. 430.)
The alteration of the conglomerate at the base of the
CHA.P. XIII.J METAMORPHIC ROCKS.
5-11
red sandstone forms another object lesson in meta-
morphism. " In its unaltered form this characteristic
hand of conglomerate is composed of more or less well-
rounded pebbles of quartz-rock, gneiss, pegmatite, diorite,
&c., embedded in a loose gritty matrix. But where it has
been subjected to mechanical movement, the softer pebbles
of gneiss and fragments of the basic dykes have been
crushed, flattened, and elongated in the direction of move¬
ment. Indeed, in some cases they have been drawn out to
such an extent as to form thin lenticular bands of micaceous
or hornblende-schist flowing round the harder pebbles of
quartz-rock. The latter still preserve their rounded form,
but they are traversed by small ' step-faults,' tending to
elongate them in the direction of movement. The original
gritty matrix has been converted into a fine micaceous, or
green chloritic schist, showing exquisite flow-structure
winding round the elongated pebbles in wavy lines. In
short, the matrix has been converted into a fine crystalline
schist, and but for the presence of the deformed schistose
pebbles it would probably be impossible to tell that the
schist had a clastic origin." (Op. cit., p. 431).
In the upper stratified series the grits and quartzites are
altered into quartz-schists, the shales are converted into
dark micaceous schists, and the limestones into marbles.
The dykes of felsite in these rocks are changed into a sort
of soft greenish mica-schist, and the diorites into horn-
blendic augen-schist and gneiss.
In the more eastern districts there are great alternations
of flaggy micaceous schist and of coarse micaceous and
hornblendic gneiss, traversed in places by bands of pink
and grey granite which have been converted into granitoid
gneiss by mechanical movements.
CHAPTER XIV.
UNCONFOEMITT AND OVERLAP.
That dry land has always existed somewhere through¬
out all geological time we know from the very exis¬
tence of mechanically-formed rocks, which are the me¬
morials of its erosion and destruction. This axiom was
well expressed by Jukes in the following passage : " Lyell
long ago showed that the amoimt of such [detrition and]
-denudation is to be exactly measured by the quantity of
the mechanically-formed aqueous rocks, and as our present
lands show us vast sheets of sandstones and clays hundreds
-and thousands of feet in thickness and hundreds and thou¬
sands of square miles in extent, and as every particle of
these enormous masses of rock is the result of the erosion
-of previously existing rocks, it follows that the amount of
denudation must have been just as great as that of deposi¬
tion. Just as when we see a large building we know that
a hole or quarry must have been made somewhere in the
earth, equal, at least, to the cubical contents of the sohd
.parts of that building ; so, where we see a vast mass of
mechanically-formed aqueous rocks, we must feel assured
that a gap was made somewhere in the surface of the earth
equal to the solid contents of those rocks." ^ Conversely,
also, when we have ascertained that a large amount of
rock material has been removed from any existing, or
any ancient land surface, we can be sure that an extensive
series of stratified deposits must have been formed some¬
where out of the materials thus removed, and that their
' Jukes' " Manual of Geology," first edition, p. 280.
CHAP. XIV.J UNCONFORMITY AND OVERLAP.
543
deposition went on pari passu with the detrition of the laud
surface.
The oscillations of level which have continually taten
place have caused frequent changes in the form of the
ancient continents, depressing some parts beneath the sea
and elevating other parts, so as to raise portions of the ad¬
joining sea-hottom into dry land. The tracts that formed
dry land were constantly exposed to the detritive action of
those agencies which were described in Part I., Chapters VI,
and VII., and the tracts that sank below ^he sea were sub¬
jected to the planing and levelling action of the sea-waves
(Chapter VIII.).
The result of both processes being to remove a great
■thickness of rock, and the special result of the latter pro¬
cess being to produce the more or less level surface which
is termed a plain of marine denudation, it follows that
when such surfaces are depressed far below the sea-level,
and newer deposits are laid down upon them, there must
be a break or discontinuity between the older rocks and
the newer deposits. The older series may form a regular
and continuous sequence of beds, but the process of depo¬
sition having been interrupted by a period of erosion and
disturbance, there must naturally be a certain amount of
discordance between the older and newer series, ßuch
discordance constitutes unconformity.
The importance of these conclusions cannot be over-esti¬
mated, and ■without a knowledge of the facts and inferences
to be described in the present chapter, the student could
make but little progress in the study of stratigraphical
geology.
Every country has its own geological sequence, com¬
prising a great series of strata, which is divisible into
many natural groups or stages, and between any two of
these groups there are only two possible relations : they
must be either in conformable or unconformable succession.
These terms we proceed to define.
1. Conformable Succession.—Two sets of beds are
said to be conformable when the base of the upper set rests
evenly upon the surface of the lower set, that surface being
everywhere one of original deposition. The plane of sepa¬
ration between two such groups simply marks an interval
544
STRUCTURAL GEOLOGY.
[part ii.
of comparatively short duration, during which no deposi¬
tion took place. Two or more conformable groups of beds
will therefore form a regular sequence like that of the
groups 1, 2, 3, 4, etc., in figs. 108 and 134.
2. Unconformable Succession.—Two sets of beds
are said to be unconformable when the base of the upper
set passes over and across the eroded edges of the lower
set, the upper surface of the latter being in fact a surface
of erosion. Such a plane of separation involves the lapse
of an interval more or less long, during which parts of the
beds previously formed were broken up and carried away,
leaving a discontinuous sequence or a discontinuity in the
stratigraphical succession, as in the lower part of fig. 172,
where the lower group of beds, m, m, have been tilted and
planed off till the horizontal surface, a, b, was produced,
on which the overlying strata 1 to 7 have been deposited.
This definition of unconformable succession is a very
general one, and will include cases which are not gene¬
rally called unconformities, though they are discontinuities.
There are, therefore, two kinds of discontinuity or irregu¬
larity, either of which may cause a break in the regular
upward succession of a series of stratified rocks ; and, as the
distinction between them involves important considerations,
it is necessary that they should both be clearly defined.
a. Contemporaneous Erosion.—This is a partial discor¬
dance caused by the erosion of channels and hollows in a
lower group of beds before the deposition of the overlying
CHAP. XIV.] UNCONFORMITY AND OVERLAP.
645
group. Thus in fig. 181, a channel has been eroded out of
the beds c, c, and the succeeding strata, s, s, have been
deposited horizontally upon the uneven surface thus pro¬
duced. In such cases the erosion has only been local and
partial, and other exposures of the two groups would show
that in most places the beds s, s, rested evenly and conform¬
ably upon the beds c, c. It is obvious, moreover, that if
the deposits were subsequently tilted, they would both be
inclined at the same angle, and would strike in the same
direction.
B. True Unconformity involves complete and widespread
discordance, and may be defined as the superposition of
one group of beds upon the upturned and eroded edges of
another group (as in fig. 180). In other words, the dip
and strike of the two groups, when both are tilted, will
be different. In such cases the interval between the two
Fig. 181. Contemporaneous Erosion. '
groups must have been long enough for great geographical
changes to have taken place ; these must have included
the elevation of the lower group, the production of a land
surface, and the subsequent depression of this surface
below the sea. Sometimes the difference of dip is so slight
that it only becomes apparent after a large area of ground
has been surveyed. In other cases the faulting of the
older beds before the deposition of the newer, as in fig.
142, or their flexure and contortion as in fig. 187, renders
the fact of unconformity easy to detect.
The occurrence, then, of a strong unconformity with a
marked discordance of dip may in itself be taken as proof
that dry land existed for some time there during the
interval between the formation of the two series of strata.
But a case of contemporaneous erosion only proves that
the lower beds have been brought within the action of
currents, without being raised above the surface of the sea
in which they were desposited.
N N
546
STRUCTURAL GKOLOGY.
[part ii.
The student must understand that, in many cases, the
existence of an unconformity between two formations can
only be ascertained by observing several points of jimc-
tion, or by mapping the country. In some cases a single
exposure, such as a cliff section, may leave no reasonable
doubt as to the relations of two formations ; but in other
cases it would be impossible to say, from the inspection of
one exposure, whether a discordance therein visible was
local or widespread. The difference of dip may not always
be discernible in a single small exposure, and the strike of
the upper formation may even coincide for a short distance
with that of the lower series. If, however, the boundary
Fig. 182. Diagram to illustrate Unconformity.
line of the upper series is followed across the coimtry, the
observer will soon ascertain whether it passes across the
outcrops of some of the members of the lower series, as it
must do if it has a different strike and dip.
Fig. 182 is intended to represent a geological model of an
area where two formations, separated by an unconformity,
are dipping in the same direction, but with a different
inclination.
It will be seen that though for a short distance between
a and b the boundary of the upper formation is nearly
parallel to the strike of the lower beds s, t, p, yet between
CHAP. XIV.J UNCONFORMITY AND OVERLAP.
547
B and c it passes obliquely across them, and is really
unconformable to them, as shown by the profile section,
where the bed c o rests on the upturned and eroded edges
of the beds o, p, s, t. c c may be taken to represent a
coarse sandstone, and abc will indicate its line of outcrop.
The basement beds of a formation which rests uncon-
formably upon another are frequently conglomerates or
pebbly sandstones ; they represent the beach deposits
which were formed along the shore lines of the old land,
and were spread out over its surface as it sank beneath
the sea, in which the newer overlying strata were accumu¬
lated. It must not, however, be supposed that an uncon-
fomity is everywhere accompanied by pebble beds ; they
will generally be found somewhere along the boundary
line of an unconformable series, but may be local and
Fig. 183. Overlap and Overstep.
ah e d e, Jurassic Kocks. 1 2 3 4 5, Cretaceous Kocks.
lenticular beds, just as the shingle beds on our own shores
are local and discontinuous deposits ; so that some parts
of the old land surface may not have any such coarse
deposits upon them.
Overlap may be described in this place, because the
occurrence of overlap is dependant on the existence of an
unconformity. The term itself, however, does not denote
any relation of a newer series to an older, but applies
merely to the relative extension of beds in a conformable
series. Overlap occurs in a conformable series when each
succeeding bed stretches beyond the limits of that below
it in one or more directions, so as to have a wider extension,
and to conceal the edges of the lower beds.
In fig. 183, it will be seen that the upper series of beds,
1 2 8 4 5, have a continually increasing extension,
2 extending further west than 1, 8 further than 2, and so
on. It is also clearly seen that the overlapping beds are
conformable to one another, though unconformable as a
548
STRUCTURAL GEOLOGY.
[part ii.
series to the rocks below. Overlap is indeed a necessary
consequence of the underlying surface of denudation being
an inclined plane instead of a horizontal one, consequently
any slope in an area of subsidence will give rise to the
phenomena of overlap.
Overstep or Transgression.—This should be dis¬
tinguished from overlap because it is a relation between
two rock-groups which are in unconformable succession ;
overlap being a relation between two groups which are
in conformable succession. Overstep may be defined as
the transgression or continuous extension of a single bed,
or group of beds, across the outcrops of an older series of
beds. It occurs when the older series has been tilted, so
as to dip steadily in one direction before the deposition of
the newer group. It is, therefore, a case of unconformity,
and may or may not be accompanied by overlap.
Thus in fig. 183 the upper series as a whole oversteps
successive members of the lower series, passing over the
basset surfaces of e d c 5, so as ultimately to rest on a.
The fact of overstep is perhaps more clearly seen in fig.
182, where the basement bed of the upper series passes
across or oversteps the basset surfaces of the beds, t s j),so
as to rest on the bed, o. In this case there is no overlap,
while in fig. 183 the upper series is an overlapping one,
and the student will see the difference by comparison.
Overlap is a relation between two members of one confor¬
mable series or system ; overstepis arelation between the base
of one series, and two or more members of an older series.
Instances of Contemporaneous Erosion.—The
subaqueous erosion of a bed can only be effected by cur¬
rents, and a surface which displays such channels and
hollows must therefore have been brought within the in¬
fluence of currents which only act powerfully in compara¬
tively shallow water. Erosion of this kind is consequently
an indication of shallow water.
Trough-like hoUows are sometimes met with in coal¬
mining, portions of the coal-seams having been removed
and the spaces filled up with clay, shale, or sandstone ; the
infilling material being usually the same as that which
forms the roof of the coal elsewhere. Mr. Buddie has
described an instance occurring in the Forest of Dean,
CHAP. XIT.J UNCONFORMITY AND OVERLAP.
549
where the miners gave thé name of the horse to the mate¬
rial which thus seemed to come down and press out the
coal. This trough was found to branch when traced over
a considerable area, and to assume all the appearance of
having been formed by a little stream with small tribu¬
taries falling into it; the channels of the stream being
afterwards filled up by the subsequently deposited materials
which were spread over the whole coal.
In other parts of the country, similar interruptions to
the continuity of a coal-seam are called rock-faults, though
of course they have nothing to do with any kind of fault
in the geological sense of the term. Fig. 184 represents an
instance which occurred in a coal-seam at Coleorton, Leices¬
tershire.^ It is part of a channel excavated in the main
coal, c, which is here about eight feet thick, and subse-
Fig. 184. Eock Fault, or Contemporaneous Erosion of a Coal-seam,
0, Main coal, s, Sandstone.
Length of section about 15 yards.
quently filled up with the sandstone, s. The component
layers of the sandstone are very irregtdar and obliquely
laminated, another result of current action (see p. 384),
which is of course very likely to occur in cases of contem¬
poraneous erosion.
Pig. 185 is an instance observed and described by Mr. J.
Shipman, near Nottingham.^ It will be noticed that the
pebble-beds, a, have been eroded into hollows and ridges
before the deposition of the white sands, 6, which are ex¬
ceedingly variable in their thickness, because they rest on
this uneven surface and have themselves suffered erosion
before or during the deposition of the conglomerate which
overlies them. This conglomerate passes up into the soft
^ See "Geology of the Leicestershire Coal-field," by E. Hull,
p. 54.
' Quoted by Aveline, " Geology of the Country round Notting¬
ham," p. 28.
650
structural geology.
[part ir.
brown sandstones, d, which are locally known as " Water-
stones."
There are cases where a considerable amount of erosion
has taken place over a large area, consequent upon some
physical and geographical change, but without any signs
of terrestrial disturbance. It is often difficult to say
whether such a case should be considered one of extensive
contemporaneous erosion, or one of slight unconformity.
A break of this kind occurs in the formation Imown as
the Lias, which strikes across the midland counties of
England from the estuary of the Severn to that of the
Humber. The Lias consists of three groups of beds called
Lower, Middle, and Upper Lias, the middle including a
ferruginous rock called the Marlsfone. In Lincolnshire
the upper surface of this marlstone was eroded to such an
extent before the deposition of the Upper Lias shale upon it,
that its thickness is now very variable, in some places 30
feet, in others only 3 or 4, and occasionally it is completely
cut out. This break has been spoken of as an unconfor¬
mity, but it is really only a case of contemporaneous erosion.
The break between the two groups known as the Great
Oolite and the Northampton Sand is of a larger and more
extensive nature, inasmuch as throughout the counties of
Northampton and Rutland certain beds are absent which
in Lincolnshire are 100 feet thick and in Gloucestershire 200
feet, and because certain small faults displace the lower
beds without affecting the Great Oolite, so that a certain
amount of terrestrial disturbance seems to have taken
CHAP. XIV.] UNCONFORMITY AND OVFRLAP.
551
place. There is a marked plane of erosion between the
two series, and the upper one has a layer of ironstone
nodules at its base (see fig. 186), yet the general strike of
the beds is the same, and there is no proof that a land
surface existed in the interval.
The break between the two members of the Cretaceous
system known as the Gault and the Cambridge Greensand
is another instance. This almost amounts to an uncon-
foijnity, for it extends over an area that is fifty miles in
length, and the Cambridge Greensand or "nodule-bed,"
which rests on an eroded surface of the Gault, contains the
riddlings of beds, which are about 100 feet thick in Bucks ;
stiU there is no difference of strike, and no evidence that
Fig. 186. Contemporaneous Erosion of Northampton Sands.
a, Northampton Sands. h, Clays of the Great Oolite.
the surface of the Gault was acted on by any other agent
than marine currents. It is really a case of contempora¬
neous erosion on a large scale, for we know that what hap¬
pened was this ; that after the formation of the Gault and
while certain beds called the Upper Greensand were being
deposited elsewhere, the area now covered by the Cam¬
bridge Greensand was washed by a strong current which
destroyed the upper part of the Gault, carrying away the
muddy portion and leaving only the sand and heavy
nodules or "coprolites" to form a basement bed to the
chalk when subsidence diverted the current, or depressed
the sea-floor below the limit of its action.
Instances of Unconformity.—1. South of Ireland.
552
STRUCTURAL GEOLOGY.
[past ii.
—We have already had occasion to mention the three great
groups of rock which enter into the structure of the south
of Ireland (see p. 434), viz., 1, the Ordovician ; 2, the Old
Red Sandstone ; 3, the Carboniferous Limestone. Between
the first and second of these there is a wide unconformity ;
but the third is always conformable to the second, though
it often overlaps it, in consequence of the small area within
which the sandstone was deposited, the Carboniferous
limestone having a much wider extension.
Fig. 187 is a diagrammatic section across Freagh Hill,
a few miles north of Thomastown, in the county of Kil¬
kenny. The Ordovician rocks, s s, were disturbed and con¬
torted, upheaved and denuded, and lastly a level surface
of planation was formed across their edges during their
depression beneath the sea. Upon this surface the Old
Red Sandstone was unconformdbly deposited, succeeded
S, Ordovician. o K s, Old Red Sandstone. C, Carboniferous
Limestone.
conformably by the beds of the Carboniferous limestone.
Subsequent elevation and denudation has removed all the
limestone from the high ground, and also the red sand¬
stone, except one or two patches of it, which now form
outliers, and has thus re-exposed the level floor of the older
rocks on which the sandstones were deposited.
The boundaries of the Carboniferous limestone and Old
Red Sandstone may be followed through the counties of
Kilkenny and Carlow, affording the most convincing proof
of the great denudation of the Ordovician tract, which took
place prior to and during the formation of the Old Red
Sandstone, even to the extent of laying bare the granite
which lay beneath these older rocks. The continued de¬
pression of this old land surface, the subsequent overlap of
the Carboniferous limestone, and its deposition on the
bare granite, are also demonstrated by the mapping of
county Kilkenny.
CHAP. XIV.J UNCONFORMITY AND OVERLAP.
553
" Fig. 188 is a diagrammatic section taken a few miles
soutli of Thomastown, where the ancient denudation had
laid bare a portion of the granite above mentioned.' The
Ordovician rocks are traversed by granite veins in this
locality, and are, near the granite, altered into micaceous
schist. The Old Bed Sandstone, on the other hand, rests
upon the granite quite undisturbedly ; it is unaltered by
that rock, and is obviously made chiefly of granite sand,
containing occasionally some granite pebbles, though not
so many of those as fragments of the slate rocks, when it
rests upon them. The granite is now readüy decomposed
and easily crumbles into sand, and did so, apparently,
quite as easily at the time the Old Bed Sandstone was
deposited upon it."
Further north in county Carlow, the sandstone gradu¬
ally thins out and allows the limestone to rest directly on
Fig. 188. Section near Thomastown, Kilkenny (after Jukes).
G, Granite. S, Ordovician. o R s. Old Ked Sandstone.
the surface of the granite. The beds of limestone dip
gently from the granite, but are not altered by it, and
were evidently deposited in the sea upon a bare floor of
granite, just as beds might now be deposited upon it if
the country were again depressed beneath the sea.^
2. Gloucestershire.—The relative position of the newer
and older rocks round the Bristol coal-field affords another
excellent instance of unconformity. The older series, con¬
sisting of the Devonian Bocks, Carboniferous limestone, and
Coal-measures, are thrown into a broad periclinal basin,
the uppermost series of coal-measures lying in the centre
of this basin (see fig. 189) ; but these older rocks are un-
conformably overlaid by a much newer series, consisting of
' The relations of this granite to the surrounding rocks have
been described in Chapter X.
' Condensed from the account given in Jukes' " Manual of
Geology."
554
STRUCTURAL GEOLOGY,
[PABT II.
red sandstones and marls (Trias), dark clays (Lias), and
oolitic limestone (Inferior Oolite), which lie nearly horizon¬
tally, and are spread over the edges of the carboniferous
series, so as to conceal their outcrops in the manner indi¬
cated in fig. 189.
The figure is a generalized section through that part of
the coal-field which lies to the N.E. of Bristol, and the
faults which traverse the older rocks are not there very
numerous or important ; in other parts of the district
there are a number of faults which break the continuity of
the older strata, but do not extend through the newer
rocks. Hence it is evident that the older series had been
bent into folds and basins, fractured by faulting, worn
down and denuded by detritive agencies, and finally planed
down to a tolerably level surface by marine erosion, before
the deposition of the newer series of deposits.
abc c b a
Fig. 189. Diagram Section across the Bristol Coal-field.
a, Devonian Rocks, b. Carboniferous Limestone, c. Coal-measures.
d, Trias, e, Lias, f. Oolite.
Comparative Magnitude of Unconformities.—
The examples given in figs. 187, 188, and 189 are cases of
very strong unconformities, the older rocks in each case
having been greatly disturbed and indurated before they
were covered by the newer series, and the gap between
them representing a long period of time during which
many thousand feet of strata were accumulated in other
parts of the British region.
The case illustrated diagrammatically in fig. 183 is an
unconformity of much less magnitude ; the disturbance of
the lower series was not great, and the plane of erosion
does not indicate the lapse of a very long period. The
magnitude of the break is of course greater at the western
than at the eastern end, but if we take it at the point
above the letter h it is measurable by only a few hundred
feet of strata.
The reader will remember, therefore, that the strength
CHAP. XlV.j UNCONFORMITY AND OVERLAP.
555
or ma^itude of an unconformity is a variable quantity,
depending on the amount of terrestrial disturbance and
subaerial erosion which took place in the interval repre¬
sented by the physical break. This is indicated by the
extent to which the older rocks were tilted, faulted, flexed
and altered before they were covered by the newer strata,
and it is measured by the thickness of the deposits which
resulted from their erosion, and were laid down in neigh¬
bouring areas.
Instances of Overlap.—An excellent instance of
overlap is to be found in the south-west of England,
where the lower members of the Cretaceous series are
successively overlapped by the upper members, as traced
from east to west. This overlap is represented dia-
grammatically in fig. 183. The several groups of which
this series is composed in the Isle of Wight have already
been mentioned (see p. 420), and they are all to be
found near Swanage, on the coast of Dorset, but when
traced westward through this county by means of the sec¬
tions exposed in the numerous coves described on p. 167,
the Wealden beds become thinner and thinner, and are
ultimately overlapped by the Lower greensand. Further
west this, in its turn, disappears, and allows the attenuated
representative of the Gault to rest directly on the Lias
clays below near Lyme Eegis. Beyond this point the Gault
itself cannot be traced, for it becomes sandy, and is indis¬
tinguishable from the Upper greensand which continues
below the chalk as far as the latter is traceable. As
already stated, the overlapping strata in this instance
also overstep successive members of the Jurassic series in
consequence of their dipping in the same direction. The
same transgression or overstep is plainly visible on a
geological map of Yorkshire, where the chalk passes across
the basset surfaces of several members of the Jurassic
system.
A good example of overlap is found in the county of
Dublin, and has been fully described by Professor Jukes,
from whose account the following is condensed. The
rock-groups which occur in this county may be grouped
into two great series—an upper and a lower,—the granite
being included with the latter, because it was intruded
556
STRUCTURAL GEOLOGY.
[PAET II.
into those rocks before the deposition of the upper
series :—
/ n J ■ ■ ÍCoal-measures,
Lower j Ordovician,
Series ] Cambrian,
aeries. | Series.
Carboniferous lime¬
stone,
Old red sandstone.
If the reader will refer to sheets 102 and 112 of the
" Geological Survey of Ireland," where the surface expo¬
sures of the different rock-groups are indicated by different
colours, and to the section, fig. 190, he will see that the beds
of the upper series, taken as a whole, rest in different
places on different parts of the lower series, and upon the
granite. The upper rocks, therefore, are completely un¬
conformable to the lower. These lower rocks rise to the
E3
Sa4t<¿J ¿line
SttM.7c
; Coal-înetLSui-es
o, Granite, c, Cambrian, s, Ordovician.
Fig. 190. Diagram section across County Dublin.
surface in four separate localities—1, in the hills south of
Dublin ; 2, at the hill of Howth ; 3, on the coast near Por-
traine ; 4, in the district west of Skerries.
The next thing that would strike us is that, although
there are so many miles of boundary to the rocks of the
upper series, yet the basement beds of that series—viz., the
Old Red Sandstone and the limestone shale—are only found
at one locality, and that near the centre of the district
(Portraine). No evidence of the existence of these base¬
ment beds was discovered, either along the southern or the
northern border of the great limestone tract.
All these facts are only explicable on the principle of
■overlap in the following way ;—Before the deposition of the
upper series, the lower group must have formed an exten-
CHAP. XIV.J UNCONFORMITY AND OVERLAP.
557
sive area of dry land, with a widely denuded and irregular
surface, as suggested in fig. 190, which was produced in
the same way as such surfaces are now produced, viz., by
the action of rain and rivers.
Eventually a movement of depression set in, and this
dry land was gradually submerged beneath the neighbour¬
ing sea. The first portion of the area which was brought
below the level of the water was the part about Portraine
(below a, in fig. 190), either because it was depressed more
rapidly, or because it was lower than the other parts of the
country. Certain beds of sand, now forming the Old Bed
Sandstone of that locality, were accumulated there, and
certain beds of black shale, forming the Lower Limestone
shale, were deposited over them as the depression con¬
tinued, These beds did not extend far to the north or
south of that locality, simply because the water ended
against the shore, within a short distance of it. As the
land, however, continued to sink, the water extended
farther and farther over it ; and the beds deposited in that
water acquired, in like manner, a wider and wider exten¬
sion, and altogether overlapped those beneath them, as re¬
presented in fig. 190. It is probable that, as a final result,
horizontal beds of coal-measures were spread over the
whole area, resting upon the granite of the Wicklow
hills on the south, and upon the hills of Meath on the
north, and completely concealing aU the limestone below.
Patches of these coal-measures are only now to be found in
the northern part of the district; having been removed from
all other parts by the detrition which has produced the
present denuded surface.
After all the beds were laid down in the manner indi¬
cated, a movement of elevation took place with its con¬
comitant results of tilting, contortion, and fracture. The
formation of a second land-surface, with its resultant
denudation, brought to light the lower beds in different
places according to circumstances, and re-exposed portions
of that still older land-surface which had been covered up
by the deposition of the Carboniferous rocks.
It is not always that the fact of overlap can be so easily
detected, and the relative age and superposition of the over¬
lapping beds so easily ascertained. Let us suppose that
558
STRUCTURxVL GEOLOGY.
[part II.
circumstances had been favourable for the formation of
sandstones over the whole surface of the sinking land, and
not only at one locality: sand and shingle beaches might
have been formed continuously along the sea margin as the
land sank, as the coasts receded, and as the sea gradually
spread over a wider area. If this had happened, sand¬
stones and conglomerates would be found resting on the
old rocks under the points c, d, as well as under a, they
would pass laterally into shale and limestone in the manner
indicated previously on p. 381, and they would also be suc¬
ceeded vertically by shale and limestone as the submergence
became deeper and deeper. Such cases frequently occur,
and are liable to lead the observer astray, unless he bear
in mind that the sandstones, etc., which might exist under
the points c and d would not be continuous and coeval with
those under a, but were contemporaneous with the forma¬
tion of pure limestone in the central part of the area. The
first-formed beds under a would be the true basement beds
of the series, but the last-formed beds would belong to the
upper portion of the series, and would have to be considered
as part of the coal-measures. If, however, the whole series
subsequently came'to be elevated again into land, tilted,
contorted, fractured, and denuded, geologists who were not
on their guard might easily suppose a succession of sand¬
stones, shales, and limestones occurring always in the same
order, and with similar characters, and belonging certainly
to the same system of beds, to be separate successive parts
of the series.
Practical Importance of the Subject.—The phe¬
nomena of Unconformity and Overlap, and the conclusions
to which they lead us, have their practical as well as their
theoretical interest ; and the whole subject ought especially
to be thoroughly understood by those who are engaged in
searching for minerals which occur in one set of rocks, and
can only be reached by boring through beds belonging to
another set. The success of such borings depends in many
cases upon the conformity or unconformity of the two sets
of rocks ; and the probable thickness of the upper set may
be miscalculated unless the possible existence of overlap is
taken into account.
Thus in county Antrim, where the Trias or New Red
CHAP, XIV.J UNCONFORMITY AND OVERLAP.
569
Sandstone rests on the Carboniferous rocks, there has long
been a feeling that it is only necessary to sink through the
sandstones in order to find the coal-measures. Since, how¬
ever, the one lies unconformably upon the other, it follows
that though in one locality the Trias may rest upon coal-
measures, in other places it may lie upon any of the beds
which come out from beneath the coal-measures ; and from
the structure of the district it appears that the chances are
something like twenty to one against the coal-measures
being found under any particular spot of the Trias.
The same is the case in the central districts of England,
where the Coal-measures rest unconformably upon Silurian
rocks, and are themselves unconformably overlaid by newer
deposits of sandstone and conglomerate. Explorations in
search of coal are from time to time undertaken in this
district, and it is of the highest practical importance that
the explorers should have a complete comprehension of the
nature of these discordances, and of the possible conse¬
quences which they involve. Such knowledge is indeed
absolutely necessary to avoid the fruitless expenditure of
large sums of money. Many thousands of pounds have
been thrown away, in this central part of England alone,
in abortive attempts to discover coal, the expenditure of
which nothing but the most complete ignorance of geology
could have rendered possible.
Again, in the Bristol and Somerset coal-fields, which
are so largely covered by newer deposits that rest uncon¬
formably upon the older rocks, although coal has been
worked in many places beneath the Triassic sandstones, it
does not by any means follow that coal-seams will always
be found below these sandstones. If a boring were made
at the foot of the Cotteswold hills in the expectation of
finding coal, the undertaking would in all probability prove
fruitless ; the bore would pass through the lias clays and
Triassic sandstones, and would then enter either the
Carboniferous limestone, or the still older rocks below it, as
indicated in fig. 189.
It will be noticed, however, that in this figure an anti¬
clinal axis is represented beneath the Cotteswold hills ; this
is hypothetical, but is considered to be a very probable oc¬
currence, so that still further east the coal-measures may
560
STRUCTURAL GEOLOGY.
[PAET II;
roll in again and form another basin beneath the newer
rocks. As a matter of fact the coal-measures have been
reached at a depth of about 1,000 feet below the surface at
Burford in Oxfordshire, and it is quite possible that several
such buried coal-fields may exist under the country between
Bristol and London.
Eecent borings in London, and at other places in
Middlesex and Hertfordshire, have shown that the newer
beds (chalk, etc.) rest unconformably upon a broad ridge
of older rocks, comprising the Silurian, Devonian, and
Carboniferous series, which appear to have a general dip
to the south, and that by this southerly dip coal-measures
which overlie the carboniferous hmestone may be brought
in to the south of London, and may underlie parts of
Surrey.
The considerations above mentioned will be sufiBcient to
demonstrate the great practical importance of an uncon¬
formity between two series of rocks.
PART III.
PHYSIOGRAPHICAL GEOLOGY.
HE first chapter of this hook treated of the earth as a
■whole, and the opinions entertained with regard to the
general structure of its mass and the condition of its in¬
terior were there given. In the chapters of Part II. the
structure and arrangement of the rock-masses which com¬
pose the superficial portion of the earth's crust were
described. We are now in a position to deal with the
problems of Physiographical Geology, or that branch of
the science which seeks to account for the existence of the
varied physical features of the earth's surface, and to ex¬
plain the manner in which hills and valleys, plains and
table-lands, continents and mountain chains, have been
gradually developed by the operation of the physical forces
which act within or upon the crust of the earth.
o o
CHAPTER I.
LAND-SCTJLPTUEE, OE THE EVOLUTION OP SUEFAOE-
FEATUEES BT THE PEOCESS OP EEOSION.
TN Part I., Chapters VI., VU., VIII., we described the
-L action of the various agencies which are continually en¬
gaged in the work of erosion and denudation ; but their
operations were treated rather from a detritive point of
view, as resulting in the collection of materials for the for¬
mation of new rocks, than as leading to the production of
new physical features.
It is clear, however, that agencies which sweep away so
much rock from the land must in time cause very great
changes in its physical features and contours. An elevated
table-land, for instance, by the unequal disintegration of
its surface, and by the erosion of valleys out of its mass,
may gradually be converted into a series of hiU ranges
more or less isolated from one another. Such a process is
fitly termed Land-sculpture, as analogous to the work of
a sculptor, who carves effigies out of solid blocks of stone.
Many persons express surprise that the formation of such
prominent features as lofty hiUs and profound valleys
should be attributed to such comparatively feeble agencies
as those above mentioned ; but it must be borne in mind
that we are naturally apt to underrate the amount of work
done by these erosive agencies, because we see that in any
period of time during which we can observe their action the
results produced are very small. On the other hand, when
we look at the magnitude of the results which have been
produced in past time, we are liable to suppose that the
agencies which have operated in former times were much
564
PIIYSIOGRAPHICAL GEOLOGY. [PAET III.
more powerful and destructive than those which are now in
action around us. " When, however, we come to reason on
the matter, we find it very difficult to imagine what these
agencies could have been if they were altogether different
from ' existing causes ' ; and equally difiBcult to suppose
that existing agencies have ever acted with much greater
intensity than at present, unless we assume the general
physical laws i)f the world to have been different from what
they are now " (Jukes).
We shall therefore take it for granted, in accordance with
the tenets of the Lyellian philosophy, that all the geological
phenomena observable among stratified rocks are due to the
same causes as those now acting in some part of the world,
or to some modification and combination of these causes,
such as we may reasonably suppose to have occurred in the
course of the earth's history.
To such seemingly insigmficant and slowly acting causes,
operating continually through long periods of time, must be
attributed all the erosion of rock which gives to elevated
land its cliffs and precipices, its hills and valleys, and all the
varied features of the earth's surface.
Share taken by different Agencies.—The detritive
and erosive agencies already described may be grouped
under two heads, according to differences in their mode of
operation, and in the results of their action.
1. Marine agencies, which act along the margin of the
land, and tend to produce an approximate level surface or
plain.
2. Subaerial agencies, wliich act over the whole surface
of the land, and tend to produce a system of valleys and
watersheds, hollows and relative eminences.
Few parts of the sea-bottom could be raised into dry
land without passing through the destructive plane of the
sea-level, and no part of the land could be submerged
without passing through the same plane, and being
exposed for a shorter or longer period to the erosive
action of the waves. These agents would wear down the
inequalities of its surface and would produce an inclined
plane, the extent and inclination of which would depend
on the slope of the sinking or rising land, and the rate at
which the movement took place. It must be remembered.
CHAP. I.J
LAND-SCULPTURE.
565
too, that a tract of land which has been submerged and then
upraised must have been twice subjected to this levelling
process, and on its second emergence would present a wide
area with a nearly level or slightly undulating surface.
Such an area has been termed a plain of marine erosion, but
would perhaps be more aptly called a surface of planation.
When once any such tract had been brought almost up to
the sea-level, only a very slight further uplift would be re¬
quired to raise a large area of it through that level, and
cause it to become dry land, so that there would be no time
or opportunity for the formation of cliffs. Slight iiregu-
larities there doubtless would be, and indeed it must be
understood that the words plain and planation are used in
a general sense to convey the idea of a surface where the
vertical height of any inequality is small as compared with
the horizontal extent of the area.
As soon as this surface produced by marine erosion is
elevated into dry land, it is subjected to the detritive
action of the subaerial agencies already described, and is
ultimately carved out into new forms of hill and valley.
In the present chapter we propose to describe the manner
in which these features are developed. Regarded as corra-
dents, or agents of land-sculpture, the subaerial agencies act
in two different ways ; the atmospheric agencies, rain, frost,
sun, and wind, act generally upon rock-surfaces, wearing
away some parts faster than others, and so developing
those which best resist their action into ridges and
eminences. Running water, on the other hand, acts along
certain lines, and excavates channels into which rain carries
the material worn from the wasting surfaces. The original
course of these channels is determined partly by the slope
of the surface over which the water commenced to run, and
partly by other conditions to be mentioned presently.
Formation of Valley Systems.—The manner in
which rills of water tend to unite and form a connected
series or river system which impresses itself on the surface,
of the country as a valley-system, is well exemplified by
the drainage of a sand or mud-flat at low tide, which has
been graphically described as follows : ^ "If we watch the
^ Jukes, " Manual of Geology," second edition, p. 105.
566
PHYSIOGRAPHICAL GEOLOGY. [PAET III.
tide receding from a flat muddy coast, we see that the mud
flat, even where no fresh water drains over it from the
land, is frequently traversed by a number of little branch¬
ing systems of channels, opening one into the other, and
tending to one general embouchure on the margin of the
mud flat, at low-water mark. The surface of the mud is
not a geometrical plane, but slightly undulating ; and the
sea, as it recedes, carries off some of the lighter and looser
surface-matter from some parts, thus making additional
hollows, and forming and giving directions to currents,
which acquire more and more force, and are drawn into
narrower hmits as the water falls. Deeper channels are
thus eroded, and canals supphed for the drainage of the
whole surface. First two, and then more, of these little
systems of drainage unite, until at dead low-water we often
have the miniature representation of the river system of a
great continent (wanting, of course, the mountain chains)
produced before our eyes in the course of a single tide, in
the very manner and by the very agent by which all river
systems on all islands and continents have been produced.
The difference between them is this only, that our islands
and continents are now above the sea, not in consequence
of the gradual fall of the water, but in consequence of the
gradual rise of the land."
A land surface does not of course exactly correspond
with the conditions of a sand-flat, it does not generally
consist of one kind of material, and it does not always
slope steadily in one direction ; there is usually some main
ridge or water-shed from which the water is thrown off in
different directions. Let us suppose a land commencing to
rise above the sea, the ridge of what may eventually be¬
come a range of mountains being the flrst part to become
dry land. As soon as this tract of dry land is established
above the reach of the waves, rain and rivers commence
their work of erosion and excavation, beginning first upon
the ridge, and extending to each piece of lower ground as
it is laid dry. It is easy to see that the first rain streams
formed on the rising ridge would run directly off down the
slopes from the crest into the sea. These would begin to
form the transverse valleys of the range. As soon as they
became a little deep, other small streams would flow into
CHAP. I.]
LAND-SCULPTUKE.
567
them from their sides ; and these, acting on any softer or
more easily destructible bands of rock, would begin to ex¬
cavate tributary valleys which would extend themselves
laterally on either side of the primary or transverse valley,
and would receive the drainage of a certain section of
the region. As the land rose on each side of the ridge,
the streams would extend their channels over it, and some
would run together and form rivers.
If the rainfall was everywhere the same, if the region
consisted of only one kind of rock, and if this rock was
composed of a succession of horizontal beds, the resulting
system of rivers and watersheds would be very symmetrical ;
but since most countries consist of many different kinds of
rocks, the beds of which are tilted in different directions,
and the rainfall varies at different places, it follows that
some streams are able to work more rapidly than others,
and so the relative areas of the drainage basins come to be
very unequal.
Conditions which determine the Position of
Hills and Valleys.—The initial courses of the streams,
and the general contour of the ground, will depend on the
following circumstances :—
1. The original slope of the surface.
2. The disposition and lie of the stratified beds.
3. The relative resistant power of the rocks, depending
partly on their hardness, and partly on their chemical
composition.
4. The presence or absence of igneous rocks.
5. The direction of any strong divisional planes, such as
those of bedding, jointing, cleavage, or faults.
Of these conditions, the second is perhaps the most im¬
portant, so much depending upon whether the beds are
horizontal, inclined, or curved, that it will be instructive to
consider each of these three cases separately.
A. Erosion of Horizontal Strata.—The simplest case which
can be imagined is that of the elevation of broad and level
tracts of country, in which the strata are for the most part
nearly horizontal. In such a case the greater part of the
surface would be occupied by one or two kinds of rock, and
the resultant features would depend chiefly upon the
manner in which these crumbled down imder the action
568 PHYSIOGRAPHICAL GEOLOGY. [PAET III.
of erosive agencies. Fig. 191 illustrates the forms into
which horizontal strata of sandstone, shale, and limestone
may be carved. The sandstone which formed the original
surface, s p, has been cut through, and portions only are
left to form the flat-topped ridges which separate the
valleys. These valleys will be wide, with long gentle slopes
where the sides consist of clay, but will have a narrow
gorge or canon at the bottom, if the stream has trenched
the underlying limestone.
Such cases are rare in Europe, but excellent instances
are to be found in eastern Egypt and in the western and
central parts of North America. The forms produced by
the action of rain and wind upon the horizontal strata of
these regions, and the excavation of profound ravines or
canons by the rivers which traverse them, have already
s - -
c
b
n
Fig. 191. Erosion of horizontal Beds.
a, Limestone. b, Shale. c. Sandstone.
been described ; but the combined effect of all the different
detritive and erosive agents in Colorado and Utah is to
produce an amount of sculpturing and a diversity of out¬
line which is truly wonderful. Dr. Geikie has given an
excellent picture of the peculiar scenery of the Uinta
Mountains in the following terms : '—
" The world can show few more impressive memorials of
the efficacy of subaerial erosion than the Uinta Mountains.
There are no structureless crystalline rocks here to deceive
us with their ruggedness. Wherever the eye turns it detects
the same long lines of horizontal stratification that serve
as a base from which the reality and amount of the erosion
may be measured. . . . OriginaUy the rocks stretched in an
unbroken sheet across the moimtains ; but in the course of
ages this continuous mantle has been enormously eroded.
Deep and wide valleys, vast amphitheatres, lofty terraced
' " Geological Sketches," Macmillan, 1882, p. 229.
CHAP. I.J
LAND-SCULPTURE.
569
alcoves, and profound gorges, fretted with an infinite array
of peaks, buttresses, pinnacles, columns, obelisks, and end¬
less forms which defy the observer to find properly descrip¬
tive names for them, have gradually been carved out of
these rocks. To gain such a vivid impression of the im¬
portance of subaerial waste in the evolution of mountain
forms was worth all the long journey in itself." Views
and descriptions of this scenery will be found in Captain
Dutton's " History of the Canon Region of Colorado " (U.S.
Survey Monograph).
B. Erosion of Inclined Strata.—Next let us suppose the
case of a slightly incUned area of planation, consisting of
different strata dipping constantly in one direction, so that
their outcropping edges form broad bands stretching across
the surface. In this case the surface features developed
by denudation will depend upon the comparative destructi-
X a V b
Fig. 192. Erosion of inclined Strata.
bility of the different rocks. Where hard and soft beds
alternate with one another, as so frequently happens, the
softer bands will be worn away more rapidly than the
harder, and the surface will be sculptured into a series of
alternate ridges and hollows, with long gentle slopes in
the direction of the dip, and steeper slopes across the out¬
crop of the hard beds. These steep slopes are called escarp¬
ments, and their general direction coincides with that of the
strike. In fig. 192, s p is the original surface of planation,
ah c are beds of hard rock, and abc are the escarpments
gradually developed by the detrition of the strata, xy z.
The broken lines show the original prolongation of the beds,
and the extent to which the escarpments have receded.
It sometimes hapi^ens that the sloping upper surface of
a hard, durable bed is completely denuded of its covering
by the atmospheric agencies, so that the slope of the ground
coincides for some distance with the sloping surface of the
570
PHYSIOGRAPHICAL GEOLOGY. [PAET III,
inclined stratum (see fig. 192). Süch ä surface is called a
dip slope, while the opposite slope may be termed the scarp
slope.
The way in which these peculiar and characteristic
features are developed, appears to be as follows :—The
first lines of drainage established on the area of planation
would form transverse valleys across the slope, from s to
p, with longitudinal tributaries along the strike of the
beds. The transverse streams would cut through hard
and soft beds alike, down to a certain base level of erosion,
and the depth of their valleys in the soft beds would neces¬
sarily depend on the depth of the ravines which they could
excavate through the hard beds. Eain falling on the tracts
of groimd that lie between these transverse valleys will
develop the outcrops of the hard beds into banks or ridges
by washing away the softer strata on each side. The ridges
so developed will form the natural limits or watersheds
between the longitudinal valleys, and as the streams
deepen these valleys the height of the ridges or escarpments
will be increased.
The channels of the longitudinal streams could not,
however, be deepened below the level of the point where
they join the main river ; each tributary, therefore, will
have its base line of erosion limited by the depth of the
main valley, and when the limit of vertical erosion has
been reached, the stream will be chiefly occupied in carry¬
ing off the material washed into it by the rain, and the
whole process of valley and scarp-making will come to a
standstill.
In the first instance, there might have been many streams
running either directly ór obliquely across the beds which
subsequently became escarpments, while the lateral tribu¬
taries may have been small and short ; but as the ridges
developed themselves and the interspaces were lowered by
detrition some of the obliquely crossing streams would be
diverted into the lateral tributaries of the more powerful
transverse streams.
The long slope behind each escarpment is the result of
the original tendency of the water to run in that direction
tiU checked by the rise of the opposite scarp-slope; thus the
rivulets flowing in the direction of the dip will generally be
chap. i.J
LAND-SCULPTURE.
571
larger than those flowing from the scarp, and the stream
resulting from their union will consequently be kept
nearer the base of the scarp-slope. Another influence
tends to keep them in this position, this is the tendency
of the tributary to shift its point of junction with the
river lower down the course of the river as the latter
deepens its channel. The features thus developed are
shown in fig. 193. The lower part of this figure represents
a section along the channel of a transverse valley, cutting
through the escarpment e, the line s s being the sea-level,
and b b indicating the bottom of the valley, and the course
of the main river : the upper part of the figure is a plan
Fig. 193. Formation of Escarpments.
of the tributary valley with its stream, r l, which has cut
down to the lowest limit of erosion at its debouchure, l.
Recession of Escarpments.—During its process of develop¬
ment an escarpment will recede from the line it originally
occupied imder the influence of those agencies which are
effecting the detrition of the whole country. Eain, frost,
and springs cause a constant succession of landslips along
the scarp-slope, and every mass of rock thus detached is
gradually disintegrated and crumbled in earth, which is
eventually washed away by the rain into brooks and
streams. By these means the scarped outcrop wiU be cut
hack further and further in the direction of the dip, its
absolute height above the sea being reduced, while its re¬
lative height above the valley in front of it is increased by
the deepening of that valley. Thus in fig. 192 it will he seen
572
PIIYSIOGRAPiriCAL GEOLOGY. [pabt iii.
that the escarpments intersected at the points abc must
haa'e started from the lines intersected at e f o, and have
receded to their present positions. This recession will go
on as long as the valleys are being deepened, but will not
be continued indefinitely.
The outline of the edge or border of the escarpment de¬
pends partly upon the relative share taken by the agencies
above mentioned, and partly on the inclination of the strata
by which it is formed. Sometimes an escarpment forms a
continuous and uninterrupted ridge, running in a nearly
straight line for a considerable distance ; sometimes its
edge is furrowed by watercourses, and hoUowed out into a
succession of deep embayments, with intervening spurs
and promontories that form more or less isolated hills.
The sinuosity of its outline generally depends upon the
presence or absence of strong springs, and the local land¬
slips to which they give rise.
This difference is well illustrated by the two great escarp¬
ments which run through Lincolnshire. Of these, the
more westerly is known as the " Lincolnshire cliff," and
forms a remarkably straight ridge, running nearly due
north and south from Lincoln to the Humber, with very
few sinuosities, and with little variation in height. This
ridge is formed by the outcrop of the upper part of the
series of clays and marlstones called the Lias, capped by
the lower beds of the Lincolnshire limestone ; the easterly
dip of the beds, and the slight thickness of the limestone
along the top of the ridge, prevent the existence of strong
or numerous spring along its western slope. The easterly
escarpment is that of the Chalk Wolds, which runs in an
irregular line from S.E. to N.W., and consists of (1) a
series of stiff clays and ironstones, succeeded by (2) coarse
sand-rock and (3) red and grey chalk. The dip is very
slight, and the thickness of chalk is considerable, so that
strong springs are thrown out sometimes at the base of
the chalk, sometimes at the" base of the sand. The action
of these springs has produced beautiful little hollows and
recesses in the face of the scarp, some of which have been
widened and extended into valleys by the rain-water drain¬
ing into them from the higher ground. The edge of the
Wold escarpment has consequently acquired a most irre-
CHAP. I.]
LAND-SCULPTURE.
573
guiar and diversified outline, which forms a remarkable
contrast to that of the " Lincolnshire chff."
0. Erosion of Curved Beds.—We have next to consider
the case of an area of curved or undulating beds, and the
influence which such curvature exercises upon the develop¬
ment of its physical features.
Let the fig. 194 represent a section through such an area,
and let us suppose the rocks to consist of shales and sand¬
stones overlying limestones, and thrown into anticlinal and
synclinal curves. The features usually presented by the
detrition and sculpture of such an area are those indi¬
cated in the figure, namely, a series of alternate ridges and
valleys, with nearly equal slopes. Usually, moreover, they
have this peeuharity, that the valleys coincide with the
anticlinals, and the hills with the synclinals, so that the
stratigraphical troughs become orographical ridges, and
p. Surface of planation. A B, Valleys coinciding with anti¬
clinals.
vice versa, which is the opposite of what might, at first
sight, be deemed probable. This arrangement, however,
is the natural result of the structural peculiarities im¬
parted to the rocks by curvature, and the consequent in¬
crease or decrease of their liability to mechanical erosion.
Where the strata have been bent into broad anticlinals,
their coherence is weakened, because they have been sub¬
jected to tension ; while in the synclinals, they have been
subjected to compression, and consequently their coherence
is increased. Lastly, where they have been thrown into
sharp curves and contortions, they have generally been
more or less compacted and indurated.
Thus, when the first stream courses were established over
such an area, the main rivers would flow from s towards p,
and the compacted rocks near s would remain as high
ground. Of the tributary streams, running more or less
574
PIIYSIOGRAPHICAL GEOLOGY. [PABT III.
along the strike of the beds, some would flow along the
anticlinal axes, and some perhaps along the synclinal axes ;
let us consider which of these would be hkely to excavate
the deepest channels. It must be remembered (see p. 166)
that wher.e rocks are freely jointed they yield more easily
along these planes than along those of bedding. Now the
principal effect of tension on the rocks of the anticlinals
would be to open the lines of jointing ; consequently, as the
stream undermined the bases of the blocks, they would sbp
down on both sides along the joint planes, and falling into
the stream would be broken up and carried away. In
the synclinals, on the other hand, the effect of com¬
pression would be to close up the joints ; and, moreover,
these planes being mostly vertical to the stratification,
would incline away from the banks of the stream, so
that the beds would not be so liable to slip down.
The drainage, therefore, will eventually be directed into
the deeper channels running along the anticlinal axes, and
the widening of these valleys by pluvial action will leave
the synclinal axes to form ridges and ranges of hills. By
a similar process of circumdetrition a peñclinal basin wiU
become an isolated hiU.
Influence of relative resistant Power.—The rela¬
tion above described as ordinarily subsisting between the
flexures of the strata and the surface features of the ground
holds good only when the rocks which formed the original
surface of planation did not differ very greatly in their re¬
sistant power. If, however, some of them possessed greater
inherent powers of resisting disintegration and detrition
than their neighbours, the latter will be worn away, and
the former will remain as hills, whether this result is or is
not favoured by the general arrangement of the flexures.
The whole process, in fact, is one of natural selection, and
the development of surface inequalities out of any tract of
country may be described as the result of selective and
differential erosion, the power of the destructive agents
being directed and modified by the capacity of the different
rocks to resist them,—the softer and weaker portions of the
surface being continually attacked and destroyed, while
the harder and more durable portions survive the detrition
and maintain their position for a longer period of time.
CHAP. I.J
LAND-SCULPTURE.
575
The law of the survival of the strongest and fittest holds
good, therefore, in physiography as well as in biology, in
the development of mountains as well as in the development
of species.
Masses of hard crystalline rock, whether igneous or
metamorphic, always form high ground, chiefly because the
interlocking crystals of which they consist are less easily
separated from one another by mechanical agencies than
the particles of more loosely compacted rocks. Bosses of
intrusive igneous rock like Mynydd Mawr (fig. 158), and
interbedded sheets of felstone like those of Snowdon, Aran,
and Arenig (p. 498), are illustrations of this law, as are also
the Malvern Hills, composed of gneissic rocks, and the
Wrekin in Shropshire, composed of several crystalline rock-
masses. Even moderately hard limestones form ridges and
escarpments when associated with clays, shales, and sand¬
stones. It is entirely a question of relative durability
or capacity of resisting the action of rain and atmospheric
agencies.
Many instances of the coincidences of the hill-ranges
with anticlinal axes, when these are formed of hard arena¬
ceous rocks, are to be found in the south of Ireland : the
Commeragh, Knockmealdown, Galtee, and Macgillicuddy
ranges, and the higher mountains of Kerry, Cork, and
Waterford generally.' In these counties the hills are
formed of anticlinal folds of red sandstones and slates,
while the valleys lie in the synclinal curves.
In these and other cases the valleys coincide with syn¬
clinal axes, because these are occupied with beds, such as
shales and limestones, which yield more easily to chemical
disintegration or mechanical erosion than the rocks of the
anticlinal folds. In Cork and Waterford the limestones have
been worn away, in spite of their hardness, because they
have less resistant power than the hard and compact sand¬
stones which form the anticlinal ridges ; for the limestones
yield to chemical action, and their material is removed in
solution as well as in suspension.
Other cases where valleys coincide with synclines may
have arisen from the fact that on the original surface of
' See " Explanations of the Sheets of the Geol. Survey of
Ireland."
576
PHYSIOGRAPHICAL GEOLOGY. [pART III.
planation the synclinal axes were occupied by shales or
limestones, and the valleys excavated along these axes may
sometimes have still been maintained as lines of drainage
after the removal of all the shale or limestone.
Transferred Drainage.—There are cases where it is
unnecessary to assume the previous existence of a plane of
marine denudation in order to account for anomalies in the
present system of drainage. The channels of the rivers and
streams may have been originally established on the surface
of some formation which has since been entirely or almost
entirely removed from the area in question. Suppose, for
instance, that a large area of flexed and folded rocks of
various kinds is unconformably covered by a newer series,
of which the highest member is a clay formation, as in fig.
195. If such an area be raised and remain for a long time
as a land surface, the system of drainage formed on the
surface a b may be in time so deeply trenched that it sinks
into the surface of the older rocks, while the newer series is
reduced to the condition of isolated patches. So long
as the general surface of the older series, c n, is protected
by the cover, the river-courses vidU be engraved on it with¬
out reference to the relative hardness or the stratigraphical
arrangement of the rocks composing it. In this way a
river system may be transfert^ from the surface of one
formation to that of another, and will be engraved on the
latter in a manner which would be very diflScult to imder-
stand if we did not replace in imagination the strata that
formed the surface on which it was first established.
The vaUey of the Bristol Avon may be explained in this
way, the course of the deep trench which it has cut in the
Carboniferous rocks, and which forms the picturesque
gorge of the Avon, having been established in the Creta-
CHAP. I.J
LAND-SCULPTURE.
577
ceous and Jurassic strata which formerly extended over the
whole district. It is probable indeed that the whole drain¬
age system of the west of England and Wales was first im¬
pressed on rocks of Secondary a^e, and that it has gradually
sunk into the surface of the older rocks as these were laid
bare by the long continued action of subaerial agencies. A
similar explanation of the drainage system of the Lake Dis¬
trict has recently been given by Mr. J. E. Marr," and the
process has been described by American geologists, who
speak of such a drainage system as " consequent," in anti¬
thesis to one which is of subsequent formation.
Connection of Valleys with Lines of Fault.—
Since the process of earth-sculpture is one of selective ero¬
sion, and the detritive agents always seize upon lines of in¬
herent weakness, it is not surprising that valleys should
sometimes coincide with lines of fault. So, also, among
rocks which are well jointed, like limestones, valleys often
coincide with the direction of the master-joints. The
streams descending from the original watershed wiU natu¬
rally follow the lines of least resistance, and therefore the
direction of transverse valleys may sometimes be determined
by master-joints or by lines of fault. Similarly, in dis¬
tricts where there is no very great difference in the com¬
parative destructibility of the rocks, the course of the
tributary valleys may be guided by any Unes of weakness
which exist.
It is quite a mistake, however, to imagine that faults gave
rise to gaping fissures which were subsequently widened
into vaUeys by the action of streams, or to suppose that
most valleys have been originated by faults because some
happen to coincide with such lines of fissure. Faults only
produce a definite feature at the surface of the ground,
when they bring yielding and durable rocks into apposi¬
tion ; such a feature is due to what may be called diffe¬
rential detrition, and is not produced by the throw of the
fault ; a valley may or may not coincide with a fault under
such circumstances, but if it should do so, its direction has
been determined by the presence of the yielding rock, and
not directly by the line of fault. Even if mural features
' "Geol. Mag.," Dec. 3, vol. vi. p. 150.
P p
578
physio(;rapiiical geology.
[part nr.
■were produced at the time of faulting, they would in most
climates be rapidly worn down and destroyed, except under
the conditions above mentioned.
The inlets of a rocky coast are often due to the presence
of such fissures, whether faults or joint-planes, along which
the waves have worked their way more rapidly than else¬
where (see p. 166), but no one supposes that the width of
the inlets bears any relation to the space between the walls
of the fault or joint, or that all inlets have been formed in
this manner because a few owe their origin to such lines
of weakness. The relative destructibihty of the rocks, and
the disposition of the strata, are far more important and
more general causes of differential erosion and consequently
of the formation of valleys.
Breaching of Escarpments and Hill Ranges by
Rivers.—It is a remarkable fact, of which England
furnishes many examples, that rivers sometimes continue
a direct course through a line of escai-pment, when we
should have expected them to have been deflected either to
one side or the other along the base of the escarpment. In
other cases, a river, after running for many miles along a
well-marked valley, suddenly turns at right angles and
passes by a deep and narrow ravine through one of the
bounding ranges of hills. After the descriptions given of
the manner in which parallel ranges of hills and valleys
have been produced, the origin of these gaps or ravines
through hills and escarpments will be easily imderstood ;
the simple fact being that the hills or escarpments did not
exist when the river, which now flows through them, first
began to excavate its channel. The course of the primary
transverse river was determined by the slope of the original
surface of planation, and it has continued to occupy the
same channel during the whole process of erosion by
which the longitudinal valleys were formed and the ridges
developed.
This explanation was first proposed by Mr. Jukes to
account for the courses of some of the rivers in the south
of Ireland, and the following is condensed from his de¬
scription of the valley of the Black water, which he selected
as an illustration. ' ' The parts of the counties of Cork and
Waterford which are here adjacent to each other consist of
CHAP. I.]
LAND-SCULPTURE.
579
a number of alternate ridges and valleys, which run with
great regularity almost exactly east and west for fifty or
sixty miles. The hills are formed of antichnal folds of red
sandstones and slates, while the valleys are occupied by
synclinal curves of thick grey limestone. The loftiest hills
of the district are the Knockmealdown mountains, which
rise to 2,600 feet above the sea. The lower parallel range
on the south is known as the Drum ridge, and between
these two ranges lies the limestone valley of Lismore,
which forms a well-marked trough, both externally and
internally. It runs in from Dungarvan Bay to Fermoy, a
distance of forty miles, and a road might be taken the
whole of that distance without ever passing over ground
much more than 100 feet above the sea.
" The Blackwater river, rising in Kerry, runs for fifty
miles down to Fermoy, and continues in the same straight
line down the Lismore valley for about twenty miles
farther, to the httle town of Cappoquin.
'• There, however, the river suddenly turns at right angles
to its former course, and instead of following the regular
east and west valley, which continues straight out to the
sea at Dungarvan Bay, it cuts through the high land of
the Drum ridges due south to Youghal Bay, which is,
moreover, five mUes farther from Cappoquin than Dun¬
garvan Bay. Why does it take this extraordinary coiirse ?
Because just to the northward of the bend are the Knock¬
mealdown mountains, and the original river ran due south
from near their summits down the slope of the original
surface towards the sea. This little transverse river would
be likely to receive a large rainfall, and would certainly
have the most rapid course ; it would, therefore, be able to
keep open its original channel through the sandstone ridge
while the surface of the limestone districts was sinking
into longitudinal valleys, and the tributary streams were
gradually increasing in length. It is evident that to what¬
ever extent these longitudinal valleys might proceed, their
streams could never cross the original transverse valley
formed by the primary river. The Blackwater, as it is
above Cappoquin, is one of these tributaries, but it has
never crossed the primary river so as to continue down the
remainder of the longitudinal valley to Dungarvan Bay,
580
PHYSIOGRAPHIC AL GEOLOGY. [PABT III.
and could never do so unless something happened to cut a
channel lower down that remaining part of the valley
deeper than the channel already cut to Youghal Bay."
In this, and other cases, the primary or transverse valleys
are due, in the first instance, to the comparative steepness
of the slope of the original flanks of the mountain range,
giving an impetus to the streams, so that channels would
be worn directly across all the bands of rock that run
parallel to the length of the range, independently of their
hardness or softness.
No stream could cut its channel in any soft band in the
higher part of its course below the level of the channel in
the next hard band of the lower part of its course. Its
channel, however, would naturally be more narrow, and its
sides more steep and precipitous, in the parts where it cut
across a hard band than where it traversed a soft one.
Moreover, the longitudinal valleys, originating in these
soft or easily destructible bands, would be wider and more
regular than the transverse valleys, and if a soft band was
very wide, and continued for many miles along the length
of the chain, it is quite possible that the longitudinal valley
formed in it, although originally a mere tributary to the
transverse valley, might ultimately be worn back along the
soft band, so as to be not only wider but far longer than
the upper part of the primary valley which lies above the
junction of the two. The river in such a longitudinal valley
would be likely to bring a greater volume of water to the
point of jimction than the stream in the transverse valley
itself might bring. In such cases the stream running
down this longitudinal valley would be very likely to be
considered the main stream, although it was originally a
mere tributary of the transverse valley.
The Wealden area of Kent and Sussex furnishes another
excellent illustration. The structure of this district is that
of a broad anticlinal curve, the arch of which has been
destroyed and carried away by erosion, so that the central
area now consists of a ridge surrounded by tracts of low
ground which are bordered on the north, west, and south
by double escarpments of chalk and greensand. Eastward,
however, there is no barrier, and the coast between East¬
bourne and Folkestone is low and flat, so that we should
CHAP. I. J
LAND-SCULPTURE.
581
naturally expect to find the chief rivers running eastward
and having their outlets along this coast. But instead of
this being the case, the principal rivers rise in the central
watershed, and flow north and south, escaping to the sea
through narrow valleys which breach the great chalk
escarpments known as the North and South Downs. The
student should not fail to inspect the geological model of
the Weald exhibited in the Museum of Practical Geology
(Jermyn Street).
The only explanation of these facts is a repetition of
that already given : marine erosion first produced a siirface
of planation across the whole district while it was being
slowly elevated, so that this original surface sloped gently
from a central line towards the north and south. The
primary streams naturally followed these slopes from the
central watershed, cutting across soft and hard beds alike,
and forming the transverse valleys. Their tributaries formed
longitudinal valleys opening into these transverse channels,
and the streams have never departed from the courses
thus established upon the original surface. Thus, through¬
out the whole process of subsequent erosion and denuda¬
tion, the Medway, the Mole, and the Wey have gradually
excavated and kept open the transverse passages through
the North Downs ; while the Arun, the Adur, and the
Ouse have in the same way formed the valleys which
traverse the South Downs.
CHAPTER II.
THE INFLUENCE OF EAETH-MOVEMENTS ON THE
FEOCESSES OF EEOSION.
IT has been stated (p. 105) that the general result of the
action of the subaërial agencies of erosion is to break
up and remove the materials of the land, the rain washing
the resulting detritus into the streams and the streams
carrying it by stages into the sea. Meantime the sea is con¬
tinually attacking the more exposed parts of every coast¬
line, and is slowly reducing the area of every country and
island which it borders or surrounds.
It is clear, therefore, that if there were no counteracting
influences, if the relative level of land and sea had never
been altered, and if the continents of the primeval world
had remained fixed and stationary, they would long ago
have been reduced to the state of shoals in a universal
ocean, and surrounded with the products of their own
degradation.
It is the instability of the earth's crust, resulting in all
probability from the existence of a liquid substratum, that
has prevented this consummation: the forces developed
beneath the crust, by continually raising some portions of
the crust and allowing others to subside, have main¬
tained a certain equilibrium between the elevations and
depressions of the earth's surface, and have constantly
provided fresh surfaces for the detritive agencies to act
upon.
It is obvious, therefore, that the power of the several
agents of erosion must be greatly influenced and modified
by the movements of the earth's crust. Portions of the
CHAP. II.] INFLUENCE OF EARTH-MOVEMENTS. 583
surface must sometimes be transferred from the sphere
of one set of agencies into that of another set ; agents
which had gradually ceased to be operative must some¬
times be given fresh scope of action ; in other cases the
power of agents in full operation must be gradually re¬
duced, and when depression is carried so far as to sink a
land surface deep beneath the sea, it is removed for a time
from the action of all destructive agencies. As a rule, we
may regard upheaval as tending to accelerate and intensify
the power of such agencies, and subsidence as tending to
diminish their power and to conserve the surfaces pre¬
viously produced by allowing them to be buried under
fresh deposits.
The influence of earth-movements in relation to the
agencies of erosion is so important a factor in the produc¬
tion of the surface features of the earth, that it merits
more definite and detailed treatment than has hitherto
been accorded to it in text-books. No explanation of the
physical features attributable to denudation and erosion
can be regarded as complete unless this relation between
the surface and subterranean forces be taken into con¬
sideration, and we propose to show its importance by
specially indicating the effects of earth-movements on each
of the three chief classes of erosive agents.
Influence on Atmospheric Agencies.—Most, if
not all the atmospheric agencies act with greater intensity
at high altitudes above the sea. It is chiefly at such alti¬
tudes that great and rapid variations of temperature occur ;
and in all temperate and tropical climes it is only on the
mountains and high plateaux that the action of frost has
much power, while in every chme rain, snow, and wind are
most prevalent where the ground rises into hills and
mountains.
It becomes evident, therefore, that the upheaval of a
region will intensify the action of all these atmospheric
agencies and accelerate the rapidity with which they do
their work of disintegration. As the land rises too, a
larger and larger area will be brought into the sphere of
their operations.
Subsidence, on the other hand, will have just a contrary
effect ; it will bring portions of the hills and mountains
584
PHYSIOGRAPHICAL GEOLOGY. [PAET III.
below the level at which such agencies are specially active >
it will generally diminish the total amount of rainfall, and
will allow a thicker and more protective vegetation to spread
up the mountain slopes.
Influence on Fluviatile Agencies.—The effect of
earth-movements in accelerating or retarding the erosive
power of streams is very great. This will be the more
apparent if we first consider the extent to which valley
erosion can be carried in a stationary area. Let us assume
a newly raised tract of country with a central plateau or
tract of hilly ground from which many streams carry off
the rainfall, and unite to form several rivers which run in
different directions towards and into the sea. Each stream
and river will continue to deepen the channel it selects
until the inclination of its bed throughout its whole course
B
Fig. 196. Diagram of a Curve of Erosion.
A B, The slope of the river channel, s s, Sea-level.
is so lowered that the current, even after heavy rains,
ceases to have any power of vertical erosion.
The slope of the channel will not be uniform, because
the power of erosion depends on the velocity of the current,
and this velocity is a product of the volume of water, and
the declivity of the watercourse. Now every tributary in¬
creases the volume of the stream, and enables it to main¬
tain a given velocity on a more gentle slope than before.
Conversely, in ascending a river the slope of the channel
may be greater above the junction of each tributary with¬
out increasing the velocity of the stream because the
volume of water is less ; in other words, the limit of
ei'osion or corrasion may be reached on a steeper slope for
every decrease in the total volume of the stream. Hence
the slope to which every stream tends to reduce its channel
approaches the form of a parabolic curve, the inclination
CHAP. II.J INFLUENCE OF EARTH-MOVEMENTS. 585
of which is constantly increasing toward the source of the
stream.
The regularity of this curve is of course affected by many
natural conditions, especially by the different resisting-
power of the rocks over which the water runs, but if it
traversed only one kind of rock, and the tributaries were
nearly equidistant, there would be a near approach to a
regular curve.
If we regard a river as consisting of as many lengths or
sections as there are important tributaries, we may con¬
sider the volume of water passing through each section
when the stream is full to be a certain average quantity,
and the slope to which the current is reducing (or has re¬
duced) its channel in that section to be an approximately
straight line with a continuous fall of so many feet or
inches per mile. This slope has been called by Professor
Powell the hase level of erosion,^ but as it is not a level, the
hase line of erosion is a better term, and the curve produced
by the combination of the several lengths or sections may
be called the curve of erosion (see fig. 196).
We see, therefore, that if a region remain stationary for
a sufiicient length of time, the streams will reach a limit in
the work of erosion, and though extraordinary floods may
occasionally enable them to continue the work, the ordinary
full stream will have ceased to deepen the channel through
which it flows. Its force will be expended partly in the
transport of detritus, and partly in the work of lateral
erosion.
Let us now consider the effect which a movement of up¬
heaval will have on such a region. It is not difficult to
perceive that its general effect will be to accelerate all pro¬
cesses of erosion, if only because it will, as a rule, increase
the rainfall on the watersheds; and by increasing the
volume of water in the streams will thereby give them a
greater velocity and power of erosion. Apart from this,
however, the upheaval may increase not only the absolute
height of the land, but the declivity or proportional slope
measured in feet per mile between a given watershed and
the sea ; and wherever the declivity is increased the streams
^ " Exploration of the Colorado River, 1875," p. 203.
586
PHTSIOGRAPHICAL GEOLOGY. [pART III.
will regain their power of vertical erosion. This result,
however, must depend on the local slopes of the tract of
land which has been added to the country by upheaval.
Let us suppose that the movement has been fairly rapid,
and that a tract of varying width has been added to the
former coast-line: in some places this tract may be so
broad and ñat that its general inclination is only equal to,
or perhaps less, than the slope of the previously formed
valleys ; the river channels will then simply be continued
across it, and the curve of erosion will only be prolonged
without being otherwise altered. In other parts of the
region, however, the slope of the newly-added tract may be
greater than of the valleys which open on to it, and in this
case the streams occuppng those valleys will recover their
excavating power, and will readjust the slope of their
channels to a deeper curve of erosion. This process will
Fig. 197. Etfect of Upheaval on Valley Erosion.
take place by degrees, rapids will be formed at the old
mouth of the valley, and if the rocks are of unequal hard¬
ness, it is probable that a succession of rapids wiU even¬
tually be produced, the site of each gradually retiring up
the river and up its tributaries till every part of the valley
system has been cut down to a lower base-bne.
The diagram (fig. 197) will assist the reader to under¬
stand the final result of this action, as viewed along the
line of the main stream or river. In this the line a b re¬
presents the slope of a watercourse or river-channel when
the sea-level coincided with the line s\ and the coast was
at the point h. If now the country was raised till the sea-
level came to correspond with the lower bne s^, and the
slope of the newly formed land was represented by the
line b s', the stream would immediately begin to deepen its
channel at b and along the slope b s" This erosion would
be carried back along the whole length of the valley until
the stream had completely accommodated itself to the
CHAP. II.J INFLUENCE OF EARTH-MOVEMENTS.
587
altered conditions, and had deepened its channel by an
amount proportional to the increased height between
and s". The amount of rock which might be removed
along the bottom of the valley is indicated by the black
part of fig. 197.
If instead of being raised the region we have imagined
were depressed, so that a portion of it sank below the level
of the sea, the ends of the valleys in this portion would
be submerged and converted into tidal estuaries, and would
be partly filled with estuarine deposits ; while even above
the tidal influence the velocity of the streams would be
lessened and erosion checked. If subsidence also diminished
the amount of the rainfall on the watersheds, the velocity
of the streams would be still further decreased, and they
would begin to deposit their detritus sooner than they had
previously done ; flood plains and alluvial levels would
consequently be formed throughout the lower parts of the
valleys. It may in fact be stated as a general rule that
wherever we find an alluvial level by the side of a stream,
we may be sure that the stream is not now deepening that
part of its valley, but has reached the local base-line of
erosion.
As nearly all rivers flow over rocks of different hardness
and cut their way back by a series of rapids and waterfalls,
it has been suggested that the position of falls and rapids
is a guide to the age of the river,^ its age being great in
proportion as these are near to the sources of the river.
With certain limitations and exceptions this is doubtless
cerrect ; but, as we have seen, an upheaval of the sea-board
may cause a new set of rapids to be developed : while,
therefore, the presence of rapids in the lower part of a river-
course does not prove the valley to be of late origin, their
limitation to the higher parts of the tributary streams is
certainly an indication of the ancient establishment of the
drainage system.
It should also be noticed in passing that the lowering of
the water-level of a large lake will have the same effect on
the streams running into it as the lifting of a whole
country has on the streams running into the sea.
^ "Quart. Journ. Geol. Soc.," vol. xxxv. p. 116.
Fig. 198. An Alluvial Level in a Siltecl-ui) Valley.
CHAP. II.] INFLUENCE OF EARTH-MOVEMENT.S. 589
From the above considerations it is evident that the in¬
fluence -which movements of subsidence or upheaval exer¬
cise upon the power of rivers to erode their channels is
most important, and the author believes that it is mainly
because the advocates of river-erosion have neglected to
insist upon this point that many persons have been unable
to see the cogency of their arguments, or to believe that
rivers have had any share in the excavation of their valleys.
It is said by these sceptics that the rivers are not now
deepening the valleys in which they run, and, therefore,
that the valleys cannot have been made by the rivers ;
some have answered this by supposing that the volume of
water in the rivers was much greater formerly than it is
now, but the real answer is that these streams have reached
their base-hnes of erosion, and consequently that observa¬
tions on their present action will not enable us to realize
how they have excavated the valleys in which they flow.
This seems to be the case with many of the large European
rivers, especially the Rhine and Moselle, whose valleys
were described on p. 147.
The southern part of England is another region where
the valleys are not now being deepened. At no remote
period of geological time the whole of the British Islands
stood at a much higher level than they do now, and during
this time erosion was very active, and the whole valley
system of the coimtry was gradually elaborated. After
this period came a succession of movements the details of
which are not yet fully known, but it is fairly certain that
the last movement was a tilt from north to south, causing
a slight elevation of Scotland and a depression of southern
England, which finally became severed from the continent.
The result of this differential movement was to check the
velocity of the English streams, and to cause the forma¬
tion of those alluvial levels or water-meadows which are
such a characteristic feature in the valleys of southern and
central England. The north of England experienced little
change of level, but in Scotland the rivers have cut
channels through the old alluvia and estuarine deposits,
and are probably still in many places deepening the upper
parts of their valleys among the hills and mountains.
We have hitherto assumed that the only movement
590
PHYSIOGRAPHICAL GEOLOGY.
[PAET III.
■capable of modifying erosive agencies was one which
changed the level of the whole country in relation to that
•of the sea ; but on a large continent local inland districts
may be affected by earth-movements, and within a single
•system of drainage a tract of country may be raised or
lowered without any alteration of level occurring on the
sea-board. Lateral pressure or volcanic action may come
into play after the establishment of a valley-system. It
may cause the ujiward bulging of certain portions of the
surface over which the streams are running, but if the up¬
ward movement is sufficiently gradual, and the main river
has sufficient velocity, this river may cut its way down
through the rising tract without altering its course, and
so long as the main stream maintains its original channel,
so long wiU all its tributaries continue in theirs. The re¬
sult will be the formation of deep gorges or cañons, the
great depth of which is due to the local upward movement
of the ground ; that is to say, the river has acted as a fixed
saw moving along one plane, while the rocks through
which it has cut its channel have been pushed upward
from below.
This is believed to be the explanation of the great cañons
of the Colorado and the Green rivers. It was stated on
p. 148 that the Green river traverses the Uinta moun¬
tains ; these mountains are formed by a single broad
anticlinal fold, exposing a thickness of 24,000 feet of rock.
In the northern cañons the beds dip to the north, and the
river, following a serpentine course, eventually reaches the
axis of the range, along which it runs in a wide and open
valley for a distance of thirty miles; then it suddenly turns
to the south and traverses the southern flank of the great
fold in another series of cañons. The American geologists
believe that the course of the river was established before
the uplift, when the rocks were nearly horizontal, and that
the elevation of the fold was so slow that it did not divert
the river from its original course. Professor Powell's
words are ; " We may say, then, that the river did not cut
its way down through the mountains from a height of
many thousand feet above its present site, but having an
elevation differing but little perhaps from what it now has,
as the fold was lifted it cleared away the obstruction by
CHAP. II.J IN^-LUENCE OF EARTH-MOVEMENTS. 591
cutting a cañón, and the walls were thus elevated on either
side. The river preserved its level, but the mountains were
lifted up ; as the saw revolves on a fixed pivot while the
log through which it cuts is moved along." '
In the same way, although the beds seen in the cañons
of the Colorado are generally horizontal, these cañons are
not cut through one great unbroken plateau, but through
a series of plateaux or great geographic tables and terraces
separated by faults or monoclinal folds. The Kaibab
plateau (see fig. 427) is one of these tabular elevations ;
and Professor Powell sums up his description of the struc¬
ture of this region in the following words : " All the facts
concerning the relation of the waterways of this region to
the mountains, hills, cañons, and cliffs, lead to the in¬
evitable conclusion that the system of drainage was deter¬
mined antecedently to the faulting and folding and erosion,
and antecedently also to the formation of the eruptive beds
[lavas] and [volcanic] cones." (Op. cit., p. 198.)
Dr. Emil Tietze ^ has also expressed the opinion that in
many cases where mountain ranges are breached by trans¬
verse valleys the upper part of the river-valley is older
than the mountain-range, and that, as the rocks of the
range were slowly raised out of the water, the river pro¬
longed its course over the land thus formed, cutting its
channel deeper and deeper as the range became higher and
higher. In this way he explains the transverse valleys
which are cut across some of the highest ridges of the
Elburz mountains in Persia ; and he believes that the
gorges of the Indus and Brahmapootra through the Hima¬
layas have been formed in the same way. These rivers take
their rise on the slopes of the Karakorum and Kuenlun
ranges, both of winch are far older than the Himalayas ;
the rivers which flowed off them would extend themselves
southward when upheaval began, and cut valleys through
the Himalayan rocks as the latter slowly rose into
mountains.
Influence of Movement on Marine Erosion.—It
has been stated that if the land remained at one level for
^ " Exploration of the Colorado River, Washington, 1875," p. 153.
' " Jahrbuch der kais.-kön. geol. Reichsanstalt. Vienna," xxviii.
p. 581. See " Geol. Record" for 1878, p. 197.
592
PHYSIOGRAPHICAL GEOLOGY. [PAET III.
a sufficient length of time it would be gradually worn away,
leaving only a submarine flat or plain, covered by a shallow
sea. Some of our present plains and table-lands have been
produced in this way, the mountains that once stood over
them having been removed by the erosion of the sea. The
destruction of Graham's Island in the Mediterranean, and
the shallow flat left in its place, illustrate this result.
We have seen, however (Part I., Chapter V.), that the
land is anything but stationary, and that every part of it is
liable to be moved upwards or downwards through the up¬
per surface of the sea. During such movements, the waves
will, of course, act upon every contour line that is succes¬
sively brought within their influence. Mr. Jukes has com¬
pared the external action of the sea to that of a fixed
engine with a horizontal planing and graving machinery,
contrived to carve into any substance, while the earth-
movements are forces acting vertically, raising or lowering
the substance through the horizontal cutting plane.
If the land is elevated, the old coast with its line of cliffs
is carried inland to a certain distance, and the new raised
tract forms a terrace or platform in front of it : if the
vertical uplift has not been very great this tract will only
be a narrow terrace like those described in Part I., Chapter
v., but if after remaining stationary for a long period of
time the region be rapidly raised through 200 or 300 feet
a broad platform will be exposed. An elevated platform
of this kind is found along the north coast of Spain, and
is thus described by Dr. C. Barrois : " The rocks of the
whole Cantabrian coast, from Galicia to Santander, form
rugged mountains, which rise like cliffs at a distance of 3
or 4 kilometres (2 to 2| miles) from the coast-line, while
the space between the sea and these mountains is a narrow
plain ; the beds being levelled as if a coast road had been
purposely constructed along the base of the mountains."
" The altitude of this plain or platform appears to be
about 200 feet above the sea, but the tracts formed of
sandstone are more elevated than those of limestone, and
the latter are higher than the tracts of schist. The
numerous torrents which descend from the mountains cut
directly across the plain, and give rise to the formation of
a great number of transverse sections The com-
CHAP. U.J IXFI.rEXCE OF EARTH-MOyEME>TS. 593
parison of this plain with the shore platform now in fjrocess
of formation shows the resemblance between them : the
denuded surface, which forms a broad terrace along the
Cantabrian shore, is nothing but an elevated marine plat¬
form, a typical example of the plains of marine denudation
described by Professor Ramsay." ^
On the other hand, if a region after standing at one level
for a certain length of time be depressed to a lower level,
the height of its cliffs is diminished, and the waves have a
smaller mass of rock in front of them ; consequently they
eat much more rapidly into the land, cutting it back at a
higher level than before, and carrying out a fresh plain of
erosion.
It will be instructive to consider a special case of de¬
pression, where the land, after having stood at a certain
level for a long period has been permanently lowered, and
Fig. 199. The effect of Depression on iSlarine Erosion.
The double lines indicate high- and low-tide levels.
then allowed to remain stationary again for a considerable
time. It is evident that, during the second period of rest,
the waves will carve out a second platform, at a higher
level than the old one formed during the first period.
Fig. 199 is intended to illustrate the result of this process.
The line c n indicates the level of high tide during the
first period, when the lower platform was formed. De¬
pression of the land then caused the sea to flow over this
platform, till its high-tide level corresponded with the line
a b. The waves continued to cut down the cliffs which
once stood over d ; but the line of non-erosion being now
at a higher level, they could not wear down the new plat¬
form to the plane of the older one, which, therefore, remains
' "Kécherches sur les Terrains Anciens des Asturies et de la
Galice," " Mem. Soc. Géol. du Nord," torn. ii. p. 619.
Q Q
594
PHYSIOGßAPIIICAL GEOLOGY, [PAET III.
as a submarine terrace, until it is covered and concealed by
newly-formed deposits.
The Isle of Wight may be cited as an instance of an
island standing upon a platform of this kind, produced
by the combined effect of subsidence and marine erosion.
The island lies entirely inside the line of 10 fathom-
soundings, so that an elevation of only 60 feet would unite
it to the coast of Hampshire, leaving a level platform,
with raised beaches all round the present shore of the
island. We may, therefore, assume that its cliffs once
stood at the outer edge of this platform, and were con¬
tinuous with those of Hampshire and Sussex, and that
they have been gradually cut back to their present position
by the erosive action of the sea.
There is good reason, moreover, for believing that this
amount of erosion does not represent the total loss of land
which has taken place on the south side of the island in
comparatively recent times. There are, at any rate, certain
peculiarities in the courses of the rivers, which suggest that
when the present system of drainage was established, large
areas of land existed over the sites of Brixton and Sandown
Bays. Mr. Codrington has pointed out that the short
streams which flow into these bays must once have had
much longer courses, and were probably tributaries of two
large rivers which ran northward into the Solent (see map).
He thus accounts for the remarkable fact that both the
Yar at Freshwater and the Sandown branch of the Brading
river rise close behind the present beach, below high-water
mark, and flow northward through the chalk hiDs. Con¬
sequently, he supposes that the present short streams are
merely the abbreviated remnants of much larger rivers, the
drainage basins of which have been entirely destroyed by
the continual encroachments of the sea.'
An instance on a larger scale is furnished by the great
submarine platform on which the British Islands them¬
selves stand. It is a well-known fact that the seas sur¬
rounding our islands are comparatively shallow, and nearly
everywhere less than 100 fathoms deep. The 100 fathom
line, in fact, starts from the Bay of Biscay, and passes out-
' " Quart. Journ. Geol. Soc.," vol. xxvi. p. 528.
,CHAP. II.J INFLUENCE OF EARTH-MOVEMENTS. 595
side Ireland, at a distance of about 50 miles from its
western coast, thence it trends north-eastward, outside the
Hebrides and the Shetland Islands.
Outside these limits the sea-bottom plunges rapidly
down to the 500 and 1,000 fathom lines, and it is therefore
supposed that the European continent at one time extended
westward as far as the boundary of this submarine plat¬
form, and that this platform was formed during periods of
subsidence, when the sea ate its way farther and farther
across the land, planing down its prominences and filling
up its hollows until the movement ceased and the present
configuration of land and water was produced. If now up¬
heaved the platform would form an undulating plateau,
like a Siberian steppe, only diversified by occasional lakes
on the sites of the deeper holes which exist on the sea-
bottom.
Marine Erosion compared with Subaerial De¬
trition.—In any comparison between the relative amount
of erosion performed by the sea, and that accomplished
by the other detritive agencies at work upon the surface
of the land, the above considerations must be borne in
mind.
At the present time, where the land is stationary or
rising, the sea can only act on the edge of the land, so that
the extent of marine erosion must be comparatively small
as compared with that effected by the subaërial agencies
which act upon every foot of its surface. Dr. Geikie has
calculated that if the sea eats away the edge of a continent
at the rate of 10 feet in a century, and subaërial agents
remove a layer one foot thick from its general surface in
6,000 years, the whole continent of Europe would be en¬
tirely worn away by the latter agencies before the sea
could pare off more than a mere marginal strip of land
150 miles in breadth.
But, as he truly says,^ " In estimating the amount of in¬
fluence to be attributed to each of the denuding agents in
past times, we require to take into account the complicated
effects which would arise from the upheaval or depression
of the earth's crust. If frequent risings of the land or
^ Geikie, "Text Book of Geology," third edition, p. 448.
596
PHYSIOGRAPHICAL GEOLOGY. [PAKT III.
elevations of the sea-floor into land had not taken place in
the geological past, no great thickness of stratified rocks
could have been formed, for the first continents must soon
have been washed away." It also follows that the areas
upheaved must have been more considerable than the areas
depressed, and therefore, even in past times, the amount
of detrition effected by rain, frost, springs and rivers,
must have always been greater than that accomplished by
the sea.
CHAPTER III.
THE ORIGIN OF SOME SPECIAL PHYSICAL FEATURES.
IN the first chapter we discussed some of the conditions
which have guided the erosive agencies in selecting cer¬
tain lines of action, and in developing certain general fea¬
tures of the earth's surface ; we now propose to pursue the
subject and to explain the origin of some of the special
features and local peculiarities which meet the eye of
the observer among mountains, highlands, and table-lands.
Mountain peaks may be defined as the sharpest forms
of watersheds, produced hj the continuous scarping of hard
rocks at altitudes where the atmospheric agencies work
with the greatest power. Their peculiar shapes depend
chiefly on the kind of material out of which they are
carved. Almost every kind of rock produces its own
special kind of mountain scenery, and the outlines presented
by peaks of granite, quartzite, schist, slate, sandstone and
limestone differ so greatly from one another that an ex¬
perienced geologist can often make a good guess at the
kind of rock which forms a mountain peak long before he
reaches it.
Granite hills are always broad, round-shouldered heights,
only here and there forming anything which can be called
a peak, these consisting of loose weathered blocks, piled
one on another, and often resembling masses of ruined
masonry. Hills of gneiss are similar, but often have steeper
slopes.
Quartzite hills always form sharp triangular ridges or
conical peaks, which often have some resemblance to volca¬
noes, but generally have steeper and straighter outlines.
Schiehallien, in Perthshire, is a notable instance of a
598
PHYSIOGRAPHICAL GEOLOGY, [PAET III.
quartzite hill ; seen from the south-east it appears as a
rocky ridge with a long gentle slope on the eastern side,
and a steep descent toward the west, but viewed from the
north-west it appears as a perfect cone, "raising its
gleaming peak of snowy quartzite to a height of 3,547
feet." ^ But " nowhere in the Highlands," says Sir Arch.
Geikie,'' " can the whole of the distinctive features of
quartzite scenery be seen on so grand a scale as among the
mountains of Islay and Jura. In the latter island the
quartz-rock rises into the group of lofty cones known
as the Paps of Jura, 2,571 feet above the sea, which
almost washes their base. The prevailing colour is grey,
save here and there where a mass glistens white as if it
were snow ; and as the vegetation is exceedingly scanty,
the character of the rock and its influence on the landscape
can be seen to every advantage."
Hard sandstones and grits, when nearly horizontal,
have also a tendency to form huge single pyramidal
mountains like Morven and Maida Pap in Caithness, and
the wonderful groups of isolated sandstone mountains
which are such conspicuous objects in the west of Suther¬
land and Ross. To quote again from Sir Arch. Geikie's
admirable descriptions of Scottish scenery, " these huge
pyramids, rising to heights of between 2,000 and 4,000
feet, consist of dark red strata so little inclined that their
edges can be traced by the eye in long level bars on the
steeper hillsides and precipices, like lines of masonry
These huge isolated cones are among the most striking
memorials of denudation to be seen in the British Islands.
Quenq^g, Canisp, Suilven, Coulmore, and the hills of Coygach,
Dundonald, Loch Maree and Torridon are merely detached
patches of a formation, not less than 7,000 or 8,000 feet
thick, which once spread over the north-west of Scotland "
(op. cit., p. 203).
When the sandstone strata have a decided dip or incli¬
nation the conical form of peak is more or less modified,
such hills generally breaking into a steep precipitous
slope on the outcrop side, and often forming what is termed
^ "The Scenery of Scotland," second edition, p. 205.
" Op. cit., p. 207.
CHAP. III.J SOME SPECIAL PHYSICAL FEATURES. 599
a mural escarpment that is so steep as to resemble a wall,
while the opposite side of the ridge falls away in a longer
slope, which tends to become a dip-slope, that is, the sur¬
face of the ground often coincides with the surface of a
particular bed for some distance.
Hard and massive limestones generally develop tall
columnar or turret-like peaks, the forms depending partly
on the horizontality or inclination of the beds, but more
especially on the lines of jointing. Sometimes the general
trend of a limestone mountain is determined by the direc¬
tion of one set of joints, while its separation into blocks
and peaks is determined by another set having a trans¬
verse direction. This is well illustrated in the case of the
Drei Zinnen, an excellent view of which forms the frontis¬
piece to this volume.^ These are peaks of hard Dolomite,
and form part of the region of the Dolomite Mountains in
the Tyrol. The view from this very spot has been thus
briefly described ;—" From Landro, Monte Cristallo, shooting
up in towers between its glaciers, is the dominating object.
Not far from the post-house, however, through a gorge
opening eastward there is an apparition of three splintered
spires (the Drei Zinnen, or three battlements) than which
in the way of mountains I know nothing stranger." '
The hard crystalline schists form mountain crests which
often present a sharp and serrated array of peaks. Such
serrated ridges occur among the gnarled schists which
tower into the mountain heights of Inverness ; and the
rough, rugged, and craggy outline which their summits
exhibit may be ascribed partly to the influence of foliation
and jointing, and partly to the unequal weathering due
to the differences of hardness and texture in the rock
material
Fig. 200 is a view in the Graian Alps, borrowed from Pro¬
fessor Bonney's " Alpine Regions," and shows a peak
composed of chloritic and micaceous schists which belong
to the upper part of the crystalline series of the Alps.
Many of the eminences, which are termed aiguilles in Swit-
' This plate is borrowed from Professor Bonney's "Alpine
Regions," and was drawn by Mr. Whyniper from a sketch by Pro¬
fessor Bonney.
' "The Dolomite Mountains," by Messrs. Gilbert and Churchill.
600
PHYSIOGRAPHICAL GEOLOGY.
[PAET nr.
zerland, consist of similar materials, as, for instance, some
of those in the Mont Blanc range, the Matterhorn, etc.
Corries and Cirques.—A corry is the head of a high¬
land glen, or valley, having steep, rugged, and often pre¬
cipitous sides, furrowed by the channels of the rain-rills
that descend from the crest of the watershed. In Wales
they are called " cwms," which is the same word as the
Fig. 200. ^'ie\v in the Graian Alps.
English " combe." In France and Switzerland the more
circular and cauldron-shaped corries are known as cirques.
Sir A. Geikie writes :—" No feature in Highland scenery
is more characteristic than the corries, and in none can the
influence of geological structure be more instructively
seen. Usually the upper part of a corry is formed by a
crescent of naked rock from which long screes of débris
descend to the bottom of the hollow. Every distinct
variety of rock has its own type of corry, the peculiarities
chap. iii.J some special physical features. 601
being marked both in the details of the upper cliffs and
crags, and in the amount, form, and colour of the screes." '
Some have imagined that corries and cirques owe their
origin and form mainly to the action of ice, but this view
is probably only engendered by the temporary tendency to
credit ice with much greater erosive power than it really
possesses. Professor Bonney has shown that Swiss cirques
cannot be attributed to any other agency than that of cas¬
cading rills of water converging toward one point, though
no doubt when once a small cirque is formed the recession
of its walls is powerfully aided by the action of frost and
other atmospheric agencies. Sir A. G-eikie has expressed
the same opinion.
Fig. 201 is a sketch by Professor Bonney of one of two fine
cirques which indent the highest part of the Bothstock
massif near Engelberg in Switzerland.^ Above the talus
heaps which lie on the fioor of this cirque rises a tier of
limestone cliffs, e, which support a thick mass of reddish
Sandy shale, r, the upper part of which is hollowed out
into small combes, c ; above this rises another tier of lime-
' " Scenery of Scotland," p. 165.
^ " Quart. Journ. Geol. Soc.," vol. xxvii. p. 315.
602
PRYSIOGRAPHICAL GEOLOGY. [PABT III.
stone cliffs, b, which form the lip of the cup-shaped
hollow. " The most remarkable thing about the cliffs,"
says Professor Bonney, " was the belt of reddish shaly rock
which was furrowed by a vast number of little gorges
which were often only a few yards apart, and occasionally
united, so that this part of the cliff really looked like a badly-
ploughed field set up on end. Down these gorges, many
of them dry in August, little rills descend from the snows
on the ledges, and in the combes above, and these have
generally made some trace corresponding with their size
on the harder limestone below, sometimes a mere stain,
sometimes a well-marked groove."
The Creux des Champs on the northern face of the
Diablerets, the Fer-à-cheval, near Sixt in Savoy, and the
Creux des Vents in the Jura, are all good instances of
large cirques, and are described by Professor Bonney in
the paper above quoted.
The existence of similar cirques and corries in the
mountains which he between the Red Sea and the valley
of the Nile,^ and where there are no traces of glacial
action, is an additional proof that they cannot be attri¬
buted to the action of glaciers.
Cols and Passes.—A col or mountain pass is a de¬
pression in the ridge of a watershed connecting two lateral
valleys with one another. Every gradation may be found,
between a mere notch in the crest of a ridge, and a pro¬
found gap or pass connecting two long and deep valleys.
The notch is formed by the recession of the heads of the
two valleys till the crest or ridge between them is worn in
to a col or gap, and, finally, if the action is continued long
enough, into a deep pass which can be used as a roadway.
The formation of such passes in the Highlands of Scot¬
land is well described by Sir A. Geikie in the following
words : ' " Among the high grounds where disintegration
proceeds apace, the gradual narrowing of ridges into sharp
nan'ow knife-edged crests, and the lowering of these into
cols or passes, can be admirably studied. Where two
' See "Geol. Mag.," Dec. 2, vol. iv. p. 477 ; confirmed by J.
Walther, "On the Denudation of Deserts," see "Nature," 1891,
p. 556.
" The Scenery of Scotland," p. 179.
CHAP. III.J SOME SPECIAL PHYSICAL FEATURES. 603
glens begin opposite to each other on the same ridge, their
corries are gradually cut back until only a sharp crest
separates them. This crest, attacked on each front and
along the summit, is lowered with comparative rapidity,
until, in the end, merely a low col, pass, or balloch may
separate the heads of the two glens."
When the ridge between the two glens has been so far
reduced that there are steep slopes up from it to the
higher part of the watershed on each side, water will
course down these slopes after heavy rains, and, spreading
more or less over the col, will help to lower it still further,
and to transport its debris into one or other of the two
glens. In this way the pass may eventually be so greatly
lowered that the two valleys are converted into one long
continuous valley, the original site of the dividing ridge
or watershed being scarcely perceptible, and only to be
detected by the fact that there is a point where the water
flows in opposite directions.
Such continuous valleys are not uncommon in Scotland
and other mountainous regions, and some of them traverse
portions of the Highlands from sea to sea. Thus the
valley of Loch Carrón is continued across Eoss-shire to
Contin and Cromarty Firth, its watershed being only
600 feet above the sea,- and only perceptible by the flow
of the water in the valley. A smaller but equally re¬
markable valley is that which runs southward from Loch
Fyne through Cowal to Holy Loch, and contains the
narrow waters of Loch Ech. It is quite possible, how¬
ever, that in these and other cases the existing watershed
does not exactly coincide with the original line of division
between two glens ; considering the great age of the Scotch
valleys, and the numerous earth movements which they
have experienced, its present position may have been de¬
termined by subsequent uplift, or by mounds of glacial
débris carried down some lateral glen and heaped across
the longer valley.
Mesas and Buttes.—These are both American terms,
though the first is of Spanish and the second of French
origin; they are constantly used by American geologists
to denote certain forms of hills which are frequent in that
country.
604
PIIYSIOGKAPIIICAL GEOLOGY. [PAET III.
Mesas are the long flat-topped tracts of high ground
which remain where a system of large valleys has been
carved out of a plateau of hard rock ; and they are most
characteristically developed where the strata are nearly
horizontal. Great sheets of basaltic lava, such as those of
New Mexico, and of Auvergne in France, give rise to the
formation of mesas.
A Butte is an isolated steep-sided hill of small super-
flcial area, and is, strictly speaking, an outlying block or
mass which has been detached from a plateau by the
erosion and removal of the intervening parts.
The edge of a plateau of horizontal or slightly inclined
rocks is generally carved by the action of rain and wind
into a series of bays and promontories, and the ends of the
latter are eventually separated so as to stand out on the
lower plain like sea-stacks off the headlands of a coast¬
line. If the uppermost layer of the plateau is harder
than those below, the butte will for a time have a flat
top, but if the beds are of nearly equal hardness the
butte will have a more or less conical form.
Pig. 202 is a view in the upper valley of the Green River,
Wyoming, U.S., showing a portion of a gently inclined
plateau which has been eroded into a series of detached
blocks and buttes. Such forms are frequent in the " Bad¬
lands " of the Western States. The group of flat-topped
mountains in British Guiana, of which Roraima is the
most conspicuous, are remnants of an ancient plateau,
and may come under the denomination of buttes. Similar
forms occur in other parts of the world, and the term may
therefore be usefully introduced into geological and geo¬
graphical nomenclature.
Gravel Ridges.—In some districts the natural drainage
of the country has altered so greatly in the course of ages,
that long banks and ridges of river gravel are found in
situations where no rivers could now flow. Long series of
such gravels occur in the neighbourhood of Cambridge.
They commence in a series of patches which occur at in¬
tervals along the sides of the long, but now dry, valleys
which run through the chalk hills. The farther they are
traced down each valley the larger do the patches become,
and the greater is the difference of level between the bottom
Fig. 202. View on the Green Kiver,
606
PIIYSIOGRAPHICAL GEOLOGY. ^^AET III.
of the present valley and the height at which the patches
of gravel occur, till, at last, they diverge altogether from
the existing line of drainage, and stream out to form long-
continued gravel-capped ridges which trend more or less in
the direction of Cambridge.^
Fig. 203 is a section across one of these ridges, near Wil-
braham ; it owes its existence as a ridge to the river-gravel
which rests upon it, and has protected the under-lying
Falboorn Fens. Road. Stream.
a
Fig. 203. Gravel Kidge near Wilbraham, Cambridge,
o. Modern gravel, b. Ancient gravel, c. Chalk.
Horizontal scale, 2 inches to a mile.
chalk from destruction ; the bottom, therefore, of the
ancient valley is here preserved, while the hills and slopes
which once formed its sides have long since been destroyed
and washed away by the wasting action of rain and springs.
The dotted lines in the figure indicate the general outline
of the ground at the time when the gravels were formed.
' See "Post-Tertiary Deposits of Cambridgeshire," p. 46, and
■" The Geology of Cambridge," Mem. Geol. Survey, p. 82.
CHAPTER IV.
peoofs of the amotint of mateeial removed from
existing surfaces.
IN this chapter we shall consider some geological features
which furnish absolute proof of the enormous amount
of detrition and denudation that has taken place in some
parts of the country ; these considerations will enable us
to realize what vast thicknesses of rock have in many
cases been removed during the process of erosion, which
has led to the development of the present physical features
of the country. They may be treated conveniently under
the following heads :—1, evidence from the outcrop of
strata ; 2, from faults ; 3, from outliers and inliers ; 4,
from mountains of upheaval.
1. Outcrop of Beds.—Where a series of beds dip one
beneath another, as in fig. 192, over a considerable breadth
of ground, it is clear that they cannot have originally
terminated in that manner, for it is the very fact of their
abrupt truncation that causes them to crop out on the
present surface. The higher beds must originally have
extended for long distances beyond their present lines of
outcrop, and a very great thickness of rock must once
have existed above the present surface.
Where a surface has been formed across anticlinal and
synclinal folds, the outcrops of the different beds supply a
means of actually estimating the minimum thickness of
rock which has been removed since the folds were formed.
Thus, in fig. 204, the distance between o and d may be
several miles, and the thickness of the beds, 2,3,4, 5, may
be respectively 100, 150, 50, and 300 feet. It is clear that
all these beds must originally have been continued over
608
physiographical geology. [part iii.
the anticlinal, a, or they could not have been brought into
the synclinal trough, s. The minimum thickness of rock,
therefore, which has been removed from the surface over
the point, a, must be 600 feet ; and if any more beds came
in with the same dip beyond d, their thickness would have
to be added to this amount.
Fig. 174, p. 519, illustrates an actual case in which this
estimate may be made ; if the thickness of the slates be¬
tween b and D be 5,000 feet, it is clear that a depth of rock
of more than that thickness must have been removed from
the top of the anticlinal fold, b, and, further, the same
thickness of rock must once have extended across the Menai
Straits, and all over Anglesea, unless the strata thinned
Fig. 204. Tliickness of Rock removed by Denudation.
proved by some of the beds being brought in here and
there by faults and synclinal curves.
In the same way Sir A. Eamsay has shown that in some
parts of South Wales a thickness of about 11,000 feet of
rock has been removed from the present surface of the
country.^ Fig. 205 is a reproduction of one of his sections
slightly altered for the purpose of making it clearer to the
student. The strong irregular line which limits the orna¬
mented part of the section represents the actual surface of
the groimd along which the section was taken, and the
horizontal line a little below it represents the level of the
sea. The central group of rocks, l. s., contains a bed of lime¬
stone which is easily recognized, and the destroyed portions
of it which originally connected the present outcrops are
indicated by dotted lines. The beds which overlie the
1 " On the Denudation of South Wales," Mem. Geol. Survey,
vol. i. p. 297.
M. C. L. O. E. S. L. S. O. E. S.
Fig. 205. Section showing the thickness of Rock removed from part of South Wales.
Reproduced by permission from Professor Green's " Physical Geology."
Scale, horizontal and vertical, one inch to one mile. S S, Sea-level. G G, Surface of the ground.
610
PIIYSIOGRAPHICAL GEOLOGY. [pABT III.
limestone on eacli side of the arch are about 1,500 feet
thick, and their upper limit is also indicated by dotted
lines. The sandstones which succeed must also have been
folded over the arch, and are in the figure represented as
coming in again on the north. They are from 4,000 to
5,000 feet thick in this district. How much of c. L. and
c. M. (the Carboniferous limestone and Coal-measures) also
extended northward and lay over the central area when the
arch was first formed we do not know, but some of these
beds did. The area shaded by parallel obhque lines is there¬
fore the least amount of rock that has been removed from
above the present surface of the ground. As the scale of
the diagram is one inch to a mile both horizontally and
vertically, the amount is easily measured, and found to be
nearly 7,000 feet. A little further west, over the Vale of
Towey, the amount removed is found to be about 11,000
feet.
In the same manner Sir A. Ramsay demonstrated that
a thickness of from 8,000 to 10,000 feet of Carboniferous
rocks had been removed from the central parts of the
Mendip Hills in Somerset before the formation of the
newer rocks which now surround them. A similar thick¬
ness of the same beds has been removed from the downs
west of Bristol, as shown in fig. 189 ; and not only so, but
a further thickness of about 3,500 feet of newer strata,
Trias, Lias, Oolites, Greensand, and Chalk, has been re¬
moved during the production of the present surface of the
country.
The south of Ireland also furnishes many good instances
of great denudation ; its geological structure has already
been partially described (see p. 552, and figs. 187,190). It
was stated that the coal-measures, which now only occur
as isolated patches, had originally a very wide extension
over this part of the country, overspreading and over¬
lapping the Carboniferous limestone, as shown in fig. 190.
So great, however, has been the loss of material during
the process of detrition and denudation by which the
present physical features have been produced, that not
only have the coal-measures been removed from the greater
part of the country, but the erosion has in many places
extended through the whole thickness of the Carboniferous
CHAP. IV.J AMOUNT OF MATERIAL REMOVED. 611
limestone, and has cut deeply into the Old Red Sandstone
below.
In some places the erosion has extended even to the
bottom of this division, so as to re-expose and trench into
the Ordovician rocks upon which the sandstones were de¬
posited. From the extent of this denudation, therefore,
it can be proved that a thickness of between 5,000 and
6,000 feet of rock has been removed from some parts of
the district.
2. Evidence of Faults.—Equally convincing is the evidence
furnished by the great dislocations which traverse every
country. Some of these are vertical displacements to
the amount of several thousand feet (see p. 450), and
yet they seldom produce any effect on the surface of the
ground. To take an extreme case as an illustration, there
is a fault traversing the Appalachian Mountains in North
America, which has a vertical throw of 20,000 feet, and
yet the surface is so planed down that a man can stride
across the line of fault, with one foot on a bed which was
originally ' 20,000 feet above that on which the other foot
rests. Another American fault, described by Powell as
running along the north side of the Uinta Moimtains in
Wyoming, has a displacement of nearly 20,000 feet, and
the same author estimates the amount of rock which has
been removed from the central axis of the Uinta fold to
be not less than 24,000 feet. Again, many of the maps
of the "G-eological Survey of Great Britain," especially
those of the coal-fields, show so many faults that they are
■covered with a network of white lines. These faults are
of various magnitudes, from a few feet to many hundred,
and yet no sign of such displacements is visible at the
surface, which may be a continuous tract of undulating
fields and meadows. In all these cases it is clear that a
thickness of rock equal to the maximum amount of ver¬
tical displacement has been removed from the surface of
the country.
3. Outliers and Inliers.—An outlier is, as the name im¬
plies, an outlying or isolated portion of a bed, which has
been separated from the main mass with which it was
' Campbell, " Frost and Fire," vol. ii. chap. li. p. 333.
612
PHYSIOGRAPHICAL GEOLOGY. [PAET III,
originally connected by erosion and denudation, just as an
island is separated from the mainland (see fig. 187, p. 552,
where outliers of Old Red Sandstone are shown).
The simplest form of an outlier is that which so often
occurs in connection with a line of escarpment, the imequal
recession of which has left a series of bays, promontories,
and outliers. It is easy to see how the neck of a promon¬
tory may be gradually worn through by the action of rain
and springs until it is converted into an isolated hUl, the
very existence of such a hill becoming a proof of the
former extension of the beds, and of the recession of the
escarpment.
Very frequently the existence of the promontory and
subsequent outlier is due to a slight synclinal curvature of
the beds, the inward dip giving them a greater power of
resistance to detritive agencies, and diminishing the liability
to landslips. So often is this the case that the very occur¬
rence of an outlier affords strong presumptive evidence of
this inward dip, although the actual inclination of the beds
may be so slight as to be inappreciable in a quarry.
Conversely, an anticlinal curvature, when denuded, often
gives rise to what is called an inlier, which may be defined
as the isolated exposure of an underlying bed amidst others
which are geologically above it.
Figs. 206 and 207 are respectively a plan and section
illustrating the occurrence of an outlier and an inlier. a a
is an escarpment fonned by successive beds of ironstone,
sandstone, and chalk ; o is an outlier of the chalk and
sandstone, resting on a promontory of the ironstone,
which is stUl connected with the main mass by a
narrow neck, i is an inlier of the sandstone and iron¬
stone, brought up by an anticlinal curve, and exposed in a
valley excavated out of the long slope of the limestone.
The section is taken along the line, s s, and illustrates the
relations of the external features to the internal structure
of the ground.
In the case represented by fig. 206, which has been sug¬
gested by the structure of the Chalk Wolds of Lincolnshire,
the outlier and inlier are caused by undulations parallel to
the strike, but they are perhaps more often produced in
nature by undulations across the strike, of the kind illus-
CHAP. IV.J AMOUNT OF MATERIAL REMOVED.
613
trated by fig. Ill ; the synclinals leading to the formation
of outliers beyond the escarpment, and inliers occurring
where the anticlinal folds happen to be crossed by some
river-valley inside the main line of escarpment.
Among the older rocks which have been thrown into
much sharper curves and contortions, outlying basins some¬
times occur containing a great thickness of beds in their
centre (see fig. 194), showing that a corresponding thick-
Fig. 206. Plan of Outlier and Inlier.
OA I
Fig. 207. Section along line s s.
ness must have been removed from the surrounding
districts.
Flexures, however, are not the only conditions which
give rise to the production of outliers and inliers. Faults,
by causing an upthrow or downthrow of beds, may bring
them into such positions that portions are afterwards
isolated by erosion. Thus, if the beds in figs. 206 and 207
were traversed by a strike-fault along the line marked p r,
instead of being bent into an anticlinal, an inlier might
still be produced by erosion, though, in that case, its
614
piiysiographical geology. [paet iii.,
western boundary would be formed by the line of fault.
An inlier so produced is represented at c m, fig. 141, p. 450.
This would be called a faulted inlier ; and an outlier, simi¬
larly separated by a fault from the main mass, is called a
faulted outlier. In such cases the inlier is always on the
upthrow side of a fault, and an outlier is always on the
downthrow side.
Fig. 208. Sketch-map of the Crich Hill District, hy H. Carr, C.E.
From Mantell's " Medals of Creation."
The arrows indicate observed dips. Scale, 1 inch to a mile.
Occasionally it happens that a piece of ground enclosed
by two or more faults has been bodily lifted up through a
certain vertical distance, and the upper beds being subse¬
quently removed by denudation, the lower beds are more
or less exposed, and form an inlier. Crich Hill, near Mat¬
lock, in Derbyshire, is an excellent example of such a
faulted inlier. The appearance and structure of this hill
are illustrated by the three following figures. Viewed
from the river Derwent, Crich Hill appears as a dome-
CHAP. IV.J AMOUNT OF MATERIAL REMOVED.
615
shaped eminence, encircled by precipitous escarpments of
less elevation than the central hill, the height of which
is about 800 feet above the sea, and is crowned by a tower
called the Stand.
The dimensions of this central hill, and its position with
regard to the surrounding escarpments, will be understood
Fig. 209. Geological Map of the Crich Hill District.
Scale, 1 inch to a mile. Dips shown hy arrows.
The broken lines are faults.
1, Limestone. 2, 4, 7, Shales. 3, 5, 6, Grits and sandstones.
by reference to the sketch-map, fig. 208, which represents
the physical geography of the district, while its geological
structure is shown in fig. 209. A comparison of these two
maps serves also to show the difference between an ordi¬
nary map or ground plan, and a geological map, on which
the rock-structure of a district or country is delineated.
From fig. 209 it will be seen that Crich Hill itself consists
616
physiographical geology. [pabt iii.
of limestone, the beds of which are bent into an anticlinal
curve, which becomes almost a periclinal at the north end
of the hill. The surrounding escarpments are formed by
the outcrop of several beds of hard grit and sandstone in-
terstratified with softer shales, while it is along the course
of the latter that the valleys of the district have been exca¬
vated. The dependence of physical features upon geolo¬
gical structure is here, therefore, excellently illustrated.
From the map (fig. 209) it is seen that the country is
traversed by two main lines of fault, which, together with
a cross fault on the north-west, completely cut off the
Crich Hill area from the surrounding district. The tri¬
angular tract bounded by these faults has been pushed
bodily upwards, or else the tracts on each side have been
let down, till the upper surface of the limestone (1) was
K. Derwent,
raised above the level of the sandstone (6), as shown in
the section (fig. 210) along the line a b. Furthermore,
during the process of erosion and denudation by which the
physical features of the country have been produced, the
whole of the beds, 2, 3, 4, 5, 6, 7, have been removed from
the surface of the limestone that forms Crich HiU, so that
this satisfies the definition of an inlier, and becomes a
measure of the amount of the - denudation that has taken
place.
The thickness of beds 2 to 7 is here about 700 feet, but
as they dip eastward they are covered by coal-measures to
a depth of nearly 3,000 feet, and all these must originally
have been continuous over the top of Crich Hill, and across
the great anticlinal axis of the Pennine Hills into Cheshire
and North Staffordshire. In the latter counties the beds
above the limestone are very much thicker, so that the
s
s
Fig. 210. Section through Crich Hill.
Along the line a b in fig. 209.
CHAP. IV.J AMOUNT OF MATERIAL REMOVED.
617
thickness of rock removed from the western slope of the
Derbyshire Hills must have been in some places more
than 8,000 feet.
4. Mountain Chains.—A mountain chain differs from a
range of hills, not only in its greater height and breadth,
but in its geological structure. We have seen that a range
of hills can be carved out of a set of gently inclined strata,
for many escarpments form ranges of hills, such as the
Cotteswolds and the Chiltem Hills ; or out of undulated
strata, as in fig. 194; or out of a single broad anticlinal
fold, like that of Derbyshire or the Mendip Hills ; or, finally,
out of any mass of rock that is harder than the surround¬
ing strata. A mountain chain, on the other hand, always
consists of a number of parallel folds, and has been so
ridged up that the oldest rocks are always found along
the central axis of the chain. The appearances are those
of great lateral compression, for—
1. The principal flexures are parallel to the length of the
chain.
2. The plication and contortion of the strata is always
greatest near and along the central axis.
3. The rocks composing the central axis generally show
signs of pressure-metamorphism.
Again, mountain ranges have generally been formed by
several successive uplifts at different periods of geological
time. Thus fig. 211 may be taken as a diagram section
through a mountain chain, a, b, c, d, being so many distinct
formations, each unconformable to the other, and belonging
to different geological periods. It will be seen that a had
been flexured and raised into dry land before the forma¬
tion of h, but was not then a mountain range ; a second
period of upheaval, initiating the range, occurred after the
formation of b ; and a third, completing the uplift, took
place in the interval between the periods of c and d.
618
PHÏSIOGRAPHICAL GEOLOGY. [PAET III,
It can often be proved that the total amount of rock re¬
moved from the central axis of a mountain range is enor¬
mous. Captain Dutton has recently estimated the amount
of rock removed from the Kocky Mountains of the Colo¬
rado region since their first upheaval, and believes it to
have averaged 10,000 feet in thickness over an area of
13,000 to 15,000 square miles in extent. The Alps date
from a still more recent period than the Rocky Mountains ;
but the compression to which the rocks composing them
have been subjected seems to have been much greater, and
therefore the total amount of vertical upheaval along their
axis has probably been greater than in the case of the
American range ; but as the average height of the Alps is
not much greater, the amount of denudation must have
been greater in their case. This inference is home out by the
facts, for the thickness of the sedimentary deposits affected
by the upheaval of the Alps, and exposed by denudation,
has been estimated at about 50,000 feet ; and this thick¬
ness may be regarded as the amount of rock removed from
the summits of the Alps since their first upheaval.
It is necessary to guard the student against supposing
that the removal of 50,000 feet of rock from a mountain
range means that the range must have been 50,000 feet
high at some period of its history. It need never have
been much higher than it now is, because the two pro¬
cesses of upheaval and degradation have gone on together,
and it was only in consequence of the repeated upheavals
that so great a thickness of rock was removed. The process
may be compared to the uprising of an iceberg out of the
water, in proportion as the top is melted away by the sim,
or to a piece of machinery, one part of which raises a
block of wood or metal, while another part planes off the
surface of the material as fast as it is brought within its
reach.
Mountain ranges, therefore, differ from other hills as
being mountains notwithstanding and in spite of the
immense amount of detrition and denudation which has
taken place over and around them.
General Conclusions.—From what has been said in
this and preceding chapters the reader will be prepared
to accept the statement that almost all the external
CHAP. IV.J AMOUNT OF MATERIAL REMOVED.
619
features of the land, hills, glens, valleys, and plains have
been produced by erosion. Their form is a sculptured
form, carved out of the rocks by the operation of all those
agencies which have been described in the earlier chapters
of this work. These agents combine to form a great sur¬
face-carving machinery, and the disturbing forces which
result in the upheaval of land are chiefly efBcient in bring¬
ing the rock within the influence of the erosive agents, and
are seldom productive of any direct effect upon the form
of the ground.
The only exceptions to the rule that the form of the
groimd is due to detrition and erosion, are to be found in
the alluvial flats and deltas of rivers, in hills of blown
sand, in areas covered by glacial moraines, and in the
cones of active volcanoes on which the rain has not yet
produced any appreciable effect.
It must be remembered, however, that in the case of
moimtain chains, though the forms of the peaks and
valleys are the work of erosive agencies, yet the existence
of the range itself is due to the force which has caused its
elevation. Fig. 211 has been given as a simplified section
across such a range, and though, of course, their structure
is in reality much more complicated, still the diagram will
serve to explain the process of their formation and to show
what is meant by hilla of upheaval compared with hills of
drcvm-detrition, which have been amply illustrated by the
preceding figs. 191, 192, 194. Detrition may have been
equally active in both cases, and indeed the amount of
material removed may have been greater in the former
than in the latter case ; but the comparative elevation of
the ground is in the latter chiefly caused by the removal
of the surrounding parts, while in the former the hills
would have been more lofty if the detritive agents had
been less active.
To sum up, therefore, the principal features of the
earth's surface may be thus classified according to their
mode of origin.
Valleys are of one kind only ; they have all been formed
by the erosive action of rain, springs, and rivers.
Plains are of two kinds : 1, plains of construction ;
2, plains of erosion.
620
PHYSIOGRAPHICA.L GEOLOGY. [PABT III.
Plains of construction are such as are formed by the
undisturbed extension of beds which retain the original
horizontality of deposition. Eiver deltas and alluvial
flats, fens, marshes, and silted-up lakes, are instances of
such plains. In this class may also be included the slightly
inclined or undulating surfaces sometimes produced by
the elevation of a shallow sea-bottom which special local
causes have exempted from the action of marine erosion ;
such are the Pampas of South America, the Tundras of
Siberia, and large tracts of Eussia and Poland.
Plains of erosion are those which have been formed by
marine erosion across the edges and outcrops of strata
without reference to their inclination, flexures, or fractures.
They are surfaces of planation formed by the march of the
sea across the country. The limestone plains of central
Ireland may be cited as an instance, and the country bor¬
dering the rivers Ehine and Moselle is another.
Hills are of three kinds: I, hills of accumulation;
2, hills of circum-detrition ; 3, hills of upheaval.
Hills of accumulation are such as have been formed by
the piling-up of materials upon the surface of the ground.
Volcanoes are the most important instances. (See plate
at the end of the volume.) Sand-hills heaped up by the
action of wind on drifting sand come under the same
category.
Hills of circum-detrition are such as have been left by
the removal of the surrounding rocks, or are isolated by
valleys of erosion.
Hills of upheaval are those formed by the elevation of a
central axis faster than it can be worn down by detritive
agencies.
CHAPTER V.
THE ORIGIN OF LAKES.
HE origin of the hollows or troughs in which water
accumulates to form lakes has long engaged the
attention of geologists, and it is now generally recognized
that such hollows have been formed in many different
ways. In the first place lakes may be divided into three
different classes, according to their position in relation to
the larger physical features of the country, as follows :—
In the first class may be placed all those lakes, whether
large or small, which do not lie in the course of a definite
and continuous valley. In the second, those which do lie
in such a valley. These two classes include by far the
larger number of lakes which exist, for the third class
only includes those which lie in the craters of extinct
volcanoes, and the origin of these being obvious, we need
not further consider them.
I. Plain and Plateau Lakes.—There are at least
three ways in which such lakes may have been formed ;
the hollows in which they lie may be due to : (I) original
irregularity of the surface ; (2) unequal elevation ; (3)
subsidence from solution of underlying rock.
I. Original Irregularity of Surface.—We know that the
sea-floors of the present day are by no means regular
slopes or plains, but exhibit ridges and reefs, hollows and
troughs, of various sizes and shapes. Some of these are
probably ancient hollows formed when the sea-floor was a
I. Plain and Plateau lakes.
II. Valley lakes.
III. Crater lakes.
622
PHYSIOGRAPHICAL GEOLOGY. [PABT III.
land surface, and not yet filled up ; but many are simply
due to the action of currents and to the unequal deposition
of material. When the sea-bottom is again raised into
land some of these hollows will be occupied by fresh water
and will become lakes.
Unequal surfaces of deposition are specially charac¬
teristic of deposits laid down by the agency of ice, whether
land-ice or sea-ice ; deposits so formed always present an
irregular hummocky surface, in the hollows of which water
naturally accumulates when the surface again becomes dry
land. Such surfaces are common all over the northern
parts of Europe and North America.
Lakes occupying hollows which have once been portions
of sea-floors are not infrequent. Thus there is good reason
to believe that the whole of Central Asia was once a sea
comparable to the Mediterranean, the lakes both salt and
fresh which now exist there being merely the deeper por¬
tions of that sea, for the creatures which live in them are
chiefly marine types. Crustacea belonging to marine
genera also exist in Lake Wener (Sweden), in Lake
Superior, and even in Lake Titicaca (Peru). The last is
12,500 feet above the sea, but several species of a small
Crustacean called Allorchestes occur in it, one of which is
identical with a form still living in the Straits of Magellan.
In all these cases the conclusion has been drawn that
they were hollows filled originally with sea-water, which
has been so gradually replaced by fresh water during the
elevation of the country that some of the marine creatures
were able to accommodate themselves to the change of
conditions.
2. Unequal Elevation.—No part of the earth's surface
Beems to have been stationary for a very long period of
time, speaking in a geological sense, and as movements
of upheaval or subsidence affect limited areas, it often
happens that elevation is proceeding in one area while a
neighbouring area is stationary or sinking ; consequently
if the two areas form part of one continental region, the
surface of this region is being slowly tilted. Such a tilt
may only increase the original slopes, but on the other
hand it may have a contrary effect, and may even reverse
the slope of the lower and more level parts of the region.
CHAP, v.]
THE ORIGIN OF LAKES.
623
It is obvious that in such a case the whole drainage of the
country will be altered, and lakes are likely to be formed
during the process.
It has recently been suggested by Mr. H. H. Howorth
that such a reversal of the drainage and general slope of
the country has taken place in Northern Asia.^ He thinks
that when the Mammoth lived in Siberia part of the Polar
Sea was land, and the Siberian rivers flowed southward
into the Central Asian Sea mentioned above ; the present
arrangement being a reversal of the drainage conse¬
quent upou the elevation of the central plateaux and the
subsidence of the Arctic part of Asia. If this view is
correct some of the Siberian lakes may have been formed
during this gradual change of slope.
8. Unequal Weathering—It was stated in Part I., Chap¬
ter VI., that in the process of weathering or disintegra¬
tion by atmospheric agencies some rocks were decomposed
to a great depth. This is especially the case with coarse
felspathic rocks, like Granite, Syenite, and Gneiss; and
Pumpelly has called attention to the fact that where such
rocks have been exposed to this weathering process for a
long time incipient rock-basins are formed. He points
out that the surface of the solid rock below the decom¬
posed portions is very irregular, here rising into mounds
and ridges, there falling into hollows and troughs, ac¬
cording as the rain-water has been directed toward certain
places and along certain lines, and as its action has been
helped by the presence or absence of vegetation, or by
differences in the durability of the rock itself. It is clear,
therefore, that if the loose and decomposed parts of such
rock were removed by any detersive agency the underlying
depressions might eventually be converted into lakes. He
specially indicates certain enclosed basins in crystalline
rocks in Asia as having been formed by the unequal decay
of the rock and the removal of the residual materials by
the wind.®
In the northern parts of both hemispheres there are
large tracts of granite and gneissic rock, and before the
' " Geol. Mag.," Dec. 3, vol. vii. (1890), pp. 5 and 438.
" "Amer. Journ. Science," third series, vol. xvii. p. 139.
624
PHYSIOGRAPHICAL GEOLOGY. [PAET III,
advent of the Glacial period or Great Ice Age, of which
mention was made on p. 156, all these surfaces must have
been more or less weathered in the manner described.
During that period many of these tracts were swept by
enormous glaciers and sheets of ice, and the higher por¬
tions of them were left in the condition of bare rocky
surfaces smoothed and rounded by the passage of ice, all
the loose rock and soil having been carried down to lower
levels. These rocky surfaces are now covered with lakes
and tarns, lying not only in the course of the valleys,
but scattered on the slopes of hills and on the backs of
broad ridges.
Such rock-tams have been attributed by Sir A. Ramsay
and other geologists to the direct action of ice, without
due consideration of the probable previous condition of
the surface, and ice has consequently been credited with a
power of excavation far in excess of that which it really
possesses. No one can doubt that the present surface of
these districts has been swept and modelled by land-ice,
but it does not follow that the hollows or rock-basins in
which the lakes lie were actually formed by the ice. In
the author's opinion the hollows, or most of them, were
pre-existent, though filled with deposits or with decom¬
posed rock ; the ice only acted as a dénudant, using that
term in its proper sense of laying bare a previously formed
surface : it was not the chisel which formed the surface,
but only the rasper which has scraped it clean. To this
point we shall recur in the sequel. We are now dealing
only with those lakes which do not lie in valleys.
The extraordinary number of such lakes and tarns in
certain parts of Scotland and Scandinavia ought to have
suggested that some previous agency had been at work.
We have Sir A. Geikie's testimony that in the north-west
of Scotland "the surface of the Archaean gneiss is so
thickly sprinkled with them that many tracts consist
almost as much of water as of land," and in illustration
he gives a map of part of the island of Lewis,' remarking
also that the tarns occur not only in the lines of drainage,
but are, as it were, scattered broadcast over the land.
' " Scenery of Scotland," second edition, p. 240.
CHAP. T.J
THE ORIGIN OF LAKES.
625
Their extraordinary abundance ceases, however, to be so
surprising if we reabze the kind of surface which would
be left on the removal of all loose and rotten rock from a
district composed of gneiss.
4. Local Solution of Rock.—There are two kinds of rock
which are eminently soluble in water, namely, Kock-salt
and Limestone, the former not even requiring the presence
of acids. Lakes are common in all regions where either
of these rocks occupy much space, whether at or below the
surface. Small lakes are known to have been formed in
Cheshire by the sinking in of the surface at certain
localities in consequence of the abstraction of the rock-
salt below.
The solution of limestones and the formation of caverns
by underground waters has been described on p. 117, and
the occasional falling in of subterranean watercourses was
also mentioned. Such falling in is especially likely to
occur where a swallow-hole or line of swallow-holes has
been established above a cavern, and when the roof has
given way an open cavity of considerable size may be left.
So long as the water running into it has a free exit this
would not form a lake ; but if the level of the country was
subsequently lowered, so that the water-level in the lime¬
stone was raised, and the fi'ee drainage impeded, lakes
might be formed in such cavities, and indeed in any part
of the limestone area which lay below the new water-level,
and the hollow once occupied by water would certainly be
enlarged by surface solution, especially where water from
peat-bogs drained into it. Such is probat ly the origin of
most of the lakes in the limestone districts of Ireland.
II. Valley Lakes.—The greater number of lakes, and
especially of smaller lakes, lie within valleys, but it is
impossible to attribute the formation of such lake-basins
to the agent which has excavated the valleys, namely,
running water; nor can we suppose that every such basin
is the site of a local subsidence. The following are the
chief causes which have contributed to the formation of
valley-lakes.
I. Deformation of Valley-floors.—During the formation
of a mountain chain the successive applications of lateral
pressure to the flanks of the chain may cause anticlinal
s s
626
PHVSIOGRAPHICAL GEOLOGY. [PAET III.
uplifts across the previously-formed valleys. We have
seen that in many cases such uplifts are so gradual that
the streams have time to cut through them ; but if the
uplift was more rapid, or if it produced a fault-cliff, or if
it occurred where the valley-slope was already very slight,
it might give rise to the formation of a lake, for every
such anticlinal ridge has its corresponding synclinal trough
behind it, and the lake would of course be formed in the
trough. Besides the lateral or longitudinal flexures, cross
flexures are often produced during the earth-movements
concerned in the uplift of a mountain range, and these
flexures will be transverse to the longitudinal valleys,
tending to the formation of lakes in such valleys.
Professor Bonney considers that all the larger Alpine
lakes have been originated by such flexures,* and his
arguments have never been answered by those who ascribe
them to the action of ice.
Again, mountain chains may be said to have their youth,
prime, and old age, and certain districts of ancient rocks
may be regarded as the wom-down stumps of old moun¬
tain chains. The valleys of such ancient ranges may, in
the course of ages, have been so bent and deformed by
the numerous subsidences and upheavals they have ex¬
perienced, that on their final upheaval to form part of the
present surface they ceased to form effective lines of
drainage, becoming merely long troughs in the midst of a
mountain region, with \ineven floors, in the hollows of
which lakes are formed. The Great Glen which traverses
Scotland from Loch Linnhe to Moray Firth, and which
contains Loch Ness and Loch Locky, is a valley-like
trough which could not be formed under existing geo¬
graphical conditions : it is doubtless a fragment of a very
old valley system established when the Highlands formed
part of a great mountain range that extended from Ireland
to Norway, and bordered a North European continent.
It appears to be part of a longitudinal valley, and its
original slope was probably to the north-west ; but its
floor has been faulted, flexed, and distorted in various
ways and at various periods.
' " Quart. Joum. Geol. Soc.," vol. xxix. p. 383, and xxx. p. 419.
CHAP. T.J
THE ORIGIX OF LAKES.
627
Many other Scotch lochs may safely be ascribed to the
deformation of ancient valleys, and the fact that rock-basin
lakes, i.e., lakes entirely enclosed by hard rocks, are most
abundant on those parts of the earth's surface which con¬
sist of the older rocks, is a confirmation of the view
that some of them are due to the accumulated efEects of
flexures and earth-movements.
2. Subsidence from Solution.—The formation of lake basins
in this manner has already been described, and it is obvious
that such hollows are just as likely to be formed in the
valleys as on the plains of a limestone or salt rock district.
Dr. Irving informs me that the extent to which the present
Alpine valleys are underlain by beds of gypsum and rock-
salt is shown by a series of models exhibited in the Geol.
Eeichsamtalt, in Vienna.
Dr. Credner mentions the case of the Visp-Thal, in
Canton Wallis, where a series of landslips accompanied by
small earthquakes continued for a whole month in the year
1855, and were caused by the undermining of the valley
by the removal of gypsum in solution. The district contains
twenty gypsiferous springs, and one of these alone brings
up over 200 cubic metres of calcium sulphate per annum.
3. Blocking of Valleys by Landslips.—That lakes have fre¬
quently been produced in this way has been mentioned on
p. 122. The lake of Derborence is known to have been
formed by huge falls of rock from the peaks of the
Diablerets (Bernese Alps), which blocked the course of the
river Liceme.
Lake Alleghe, in South Tyrol, was formed by a landslide
from Monte Pizzo in 1771 which heaped itself across the
valley below, and barred the course of the stream. The
Lago Nuovo, near Primiero, in South Tyrol, is another in¬
stance, the faU of rock coming from the Cima d'Asta on
the western side of the valley.'
The lakes produced by the landslips which fell during
the Calabrian earthquakes of 1738 have been mentioned
on p. 56 ; and evidences of the former existence of lakes
may be seen behind several of the more ancient landslips
in the Alps, that at Flims, for instance.
' " The Dolomite Mountains," by Gilbert and Churchill, p.451-
628
PHYSIOGRAPHICAL GEOLOGY. [PAET IIL.
4. Blocking of Valleys hy Moraines.—The moraines left
by glaciers (see p. 154) often form a complete bar or dam
across a valley, and on the disappearance of the ice they
naturally give rise to the formation of lakes, the size of
which depends on the width of the valley and the height
of the moraine. Hundreds of such lakes occur in every
hilly country which has formerly been occupied by glaciers,
and sometimes several may be found in the course of one
vaUej'; they occur also at the bottom of corries among the
Fig. 212. Llyn Cwellyn, a Lake in a drift-filled Valley
near Snowdon.
mountains, the mouth of the corrie being blocked by
moraine as the ice dwindled in its final recession.
Loch Skene, at the head of Annandale in Dumfries, is
one of the best examples in the British Isles, the glen in
which it lies being blocked by a wonderful dam of moraine
mounds through which the issuing river makes its way.''
Travellers in Norway will find a fine example in the Odde
Fiord, the lake lying behind and above a vast moraine,
which stretches right across the valley, and is over 400
feet high in the middle. Many other instances occur in
that country, as well as in Scotland and Wales.
^ See views and description in Geikie's " Scenery of Scotland,"
p. 317.
CHAP. V.J
THE ORIGIN OF LAKES.
629
It is not always easy to ascertain whether a lake is a
rock-basin, or wholly or partially upheld by a moraine dam.
Thus Lake Constance (the Boden See) was regarded by
Eamsay as a rock-basin, but it has recently been ascer¬
tained that there is a mass of glacial detritus beneath the
town of Constance, and that in all probability the lake-
bottom is a moraine-blocked valley, or at any rate that
the moraine helps to hold up the water. The Achensee in
North Tyrol is another curious case ; here the pre-glacial
valley has been completely blocked by the glacial debris
which has been brought down a lateral valley, and now
forms the pass of Mäurach at the south end of the
lake, only about 40 feet above its surface. This accumula¬
tion is believed to have reversed the drainage, the present
outlet of the lake being at the north end, while the
ancient stream flowed southward through the Jenbach
gorge into the river Inn."^
6. Erosion of Bock-hasins by Ice.—Sir A. Eamsay's
theory, that many lake-basins have been actually excavated
out of solid rock by ancient glaciers, has been previously
mentioned (see p. 159 and p. 624), and it was stated that
the arguments by which the theory was supported cannot
be regarded as conclusive. The case for the agency of ice
cotdd not be put better than in the cautious and guarded
words of Sir A. Geikie, who says " that glaciers have
occupied the glens where these lakes exist, and have worn
down the rocks along the sides and bottom of the cavities,
cannot be doubted ; but whether the ice would be capable
by itself of eroding hollows so deep as many of these lakes
is a question which has been answered with equal confi¬
dence affirmatively and negatively." He thinks it proved
that "to some extent at least the rock-basins [of glen-lakes]
have certainly been eroded by ice," and that when all the
circumstances of their position are considered, "one cannot
but feel, though the problem is not wholly solved, that
rock-basins are inseparably interwoven with the glaciation
of the regions in which they occur." " With these remarks,
so far as they refer to the deepening of glen-lakes,
' See Penck, " Glaciation of the Northern Alps."
^ " Scenery of Scotland," second edition, 1887, pp. 231-234.
630
physiographical geology. [paet iii.
everyone can agree, and it is also quite possible, as he
suggests on a later page, that in a valley which traverses
an alternating series of hard and soft rocks, the glacier
which occupied it may have worn down the soft rocks
below the level of the channel made by the stream it dis¬
placed, and under such conditions may have actually have
excavated a shallow rock-basin.
While, therefore, it may be admitted that under very
favourable circumstances a glacier may, perhaps, have
originated a shallow rock-basin, there can be no doubt
that Sir A. Ramsay pressed his theory too far. It is
generally admitted that in confined valleys glaciers are
powerful detrusive agents, and are quite capable of sweep¬
ing out any loose detritus or decomposed rock which lies
in their path, and this is quite sufficient to account for the
phenomena presented by areas where lakes abound. In
every country the pre-glacial surface must have supported
many lake-basins of ancient date, which were more or less
filled with sediment, and also many tracts of rock which
had been deeply but unevenly weathered ; all these would be
cleared out by the glaciers as they increased in size and
weight, and when the ice-age had passed away they would
be left as clean-swept basins and depressions bearing all
the well-known marks of the passage of ice. As already
mentioned, this seems to have been the history of most of
the lakes and tarns that abound in glaciated regions.
Rote.—It has recently been shown by Dr. J. W. Spencer
that the great lakes of North America owe their origin in
the first instance to the blocking of ancient valleys by
Boulder-clay; but that their present levels and contours
are due to a subsequent unequal elevation of the region,
the uplift being greater to the north-east. Dr. Spencer
thinks that Lake Erie was formed entirely by this differen¬
tial uplift.'
' See "Quart. Journ. Geol. Soc.," vol. xlvi. p. 523, and "Geo!.
Mag.," 1891, p. 262.
CHAPTER VI.
THE FORMATION OP CONTINENTS AND MOUNTAIN CHAINS.
HAYING- in the previous chapters explained how the
surface of the land is sculptured into hill and valley,
peak and pass, we may conclude by approaching a more
difficult subject, namely, the manner in which the conti¬
nents themselves, and the mountain chains which form
part of them, have been raised into their present positions.
This will take us out of the sphere of observational geology
and carry us into the realm of speculative science, where
conclusions can only be regarded as matters of fair pro¬
bability.
General Form of Continents.—Let us, in the first
plàce, glance at the general arrangement of the larger
features of the earth's surface. It is well known that
nearly three-fourths of this surface are depressed below the
rest, and are covered by salt water. These depressions are
called Oceanic basins, and the intervening areas of dry land
are termed Continents or Continental plateaux. Certain
portions of the continents rise to a still greater elevation,
and form long ridges or mountain chains.
It has been pointed out by Professor Dana and others
that the arrangement and relative position of oceans, con¬
tinents, and mountain chains is peculiar and significant.
The facts may be stated as follows, and may guide us in
considering the probable origin of these great features.
1. Mountain ranges are chiefly developed along the
borders of continents, and enclose basin or trough-shaped
areas.
2. The highest mountain ranges face the deepest parts
of the oceans.
632
PHYSIOGRAPHIC AL GEOLOGY. [PAET III.
3. All mountain ranges are formed of rocks whicli have
been subjected to enormous lateral compression.
In support of the first proposition we may take a
brief glance at the larger surface-features of the great
continents.
North America has the ranges of the Rocky Mountains
and Sierra Nevada on the west, the Alleghany range on
the east, with the great plains of the Mississippi basin
between them.
South America has the great chain of the Andes all along
its western border, the Brazilian ranges on the east, and
other mountains along the northern border, while the cen¬
tral portion consists of low plains traversed by gi'eat rivers.
A transverse section across the centre of this continent,
from Chili to Parana in South Brazil, would appear as in
Fig. 213. Diagrammatic Section through South America.
a, Andes, b, Brazilian highlands.
Africa has ranges of mountains parallel to all its coasts.
The interior of the southern half of the continent consists
of vast plains, with an elevation of from 3,000 to 4,000 feet
above the sea, and bounded on all sides by mountain
chains. The northern half is of similar construction, but
the average level of the Sahara desert is much less, and
large parts of it are between 100 and 500 feet above the
sea.
The Europasian continent does not present such regularly
disposed ranges of mountains. Nevertheless we find the
Scandinavian chain on the north-western border, and a
succession of lofty ranges traversing its southern regions—
viz., the Pyrenees, Alps, Balkans, Caucasus, Hindoo-Koosh,
and Himalayas. The northern part of the continent presents
a series of great plains which nowhere rise to more than
2,000 feet above the sea except in the Oural Mountains and
in the Scandinavian range.
Australia consists chiefly of plains of low elevation, with
fig. 213.
a
b
„mrrtttnuntunmnr^
CHAP. VI.] THE FORMATION OF CONTINENTS.
633
a range of mountains along its eastern border, a lesser
range of bills on the west side, and a central swell which
reaches to between 3,000 and 4,000 feet above the sea.
The second statement, that the deeper parts of the oceans
are fronted by high mountain ranges, is supported by the
following facts ;—
The Pacific is at once the largest and the deepest ocean ;
the northern part of it has an average depth of about
3,000 fathoms, or 18,000 feet, and the southern part one
of about 13,200 feet. It is surrounded by a nearly con¬
tinuous set of mountain chains : on its eastern side are the
Andes and the Rocky Mountains, the former including
some of the highest summits in the world, and the average
height of its crests being about 12,000 feet. The average
height of the Rocky Mountain crests is about 10,000 feet ;
and the ocean outside both plunges rapidly down to over
12,000 feet, so the total average height of the slope is
from 22,000 to 24,000 feet.
The mountains of eastern Asia do not rise to such great
heights above the sea as those of America, but there is
good reason to regard Kamskatka and the Japanese Islands
as parts of a submerged mountain chain ; it is remarkable
that immediately outside these islands there is a long tract
where the ocean is over 4,000 fathoms deep, so that the
slope from the ocean depths up to the Asiatic plateaux is
about 26,000 feet high. The Philippines, New Guinea,
and eastern Australia, seem to form a continuation of this
submerged range ; they contain mountains which rise into
peaks of 7,000 to 13,000 feet, while the ocean outside them
sinks to about 2,000 feet.
The Atlantic Ocean is the next in size ; it is divided
longitudinally by a ridge over which the average depth of
water is only 1,500 fathoms, while on each side it sinks
into troughs which have a mean depth of 2,500 fathoms.
The mountain ranges around it, however, are comparatively
low. The Brazilian highlands reach to about 6,000 feet,
the Appalachians to about the same height, and the average
height of the West African ranges is from 5,000 to 7,000
feet. The slopes from the Atlantic troughs up to these
mountain ranges have consequently an average height of
21,000 feet, notwithstanding the low height of the ranges.
634
PHYSIOGRAPHICAL GEOLOGY. [PABT III.
The Indian Ocean has an average depth of about 2,200
fathoms, or 13,200 feet. It is bordered on the west by
the high ranges of East Africa, which have an average
height of 6,000 to 8,000 feet; it is faced on the north
(India only intervening) by the largest area of lofty land
in the world, the great plateau of Central Asia, with the
range of the Himalayas, of which the average crest height
is nearly 15,000 feet. On the north-east is the Malay
Peninsula and Archipelago, apparently a submerged range,
but many peaks of which still rise to 12,000 feet, while
the ocean near them sinks to 3,000 fathoms, or 18,000
feet.
In contrast to these high ranges and deep ocean troughs
we have the Arctic Ocean, of which the average depth is
only 1,000 feet, and of which large parts are less than
6Ö0 feet, these shallow waters being bordered by the low
plains of Asia and Europe. It is noticeable, too, that the
only deep part of this ocean is opposite the highest part
of Northern Europe, namely, the Scandinavian h^hlands.
Summary of Facts to be accounted for.—The evi¬
dence of the third statement made on p. 632 has been given
on p. 617, and the facts to be accounted for may be summed
up as follows.
The great oceans are deep basins, the deepest parts of
which often run parallel to the borders of the continents.
The continents are high plateaux, the mean height of the
land above the sea, according to Dr. J. Murray, being
2,200 feet, and 16,800 feet above the mean depth of the
ocean-floor. The chief mountain ranges of the world
run along the borders of the continents and face the
deepest parts of the oceans ; the total average height of
the slopes from these ocean depths to the mean crest height
of the mountain chains is from 20,000 to 26,000 feet.
The structure of the mountain chains shows them to be
portions of the crust which have been ridged up by lateral
compression, so that they always consist of bent and con¬
torted strata (see p. 617, and fig. 211).
Further, moimtain chains appear to have roots—that is,
the crust appears to be thicker beneath them than it is else¬
where. Tlxere are two considerations which lead to this
conclusion : A. The fact that moimtain chains attract the
chap. vi.J the formation of continents. 635
plumb-line, but in a degree less tban is proportionate tO'
their visible mass. It has been calculated how much
attraction ought to be attributed to the mass of the Hima¬
layas, and it was found that they ought to have attracted
the plumb-line more than they actually did. Sir Gr. B.
Airy has explained this anomaly by the supposition that ■
there is a still greater protuberance of rock downwards
into a denser yielding substratum than there is upward
into the air. b. That pointed out by Mr. O. Fisher, that
the downward increment of temperature is less beneath
mountains than beneath plains, as it should be if there
were a greater thickness of rock beneath the former.'
We have now to consider the explanations which have
been put forward to explain the facts above stated.
The Hypothesis of Secular Contraction.—We
saw, in Part I. Chapter I., that there was good reason for
believing that the earth has always been losing heat by
dissipation into space. Now it has been ascertained by
experiment that most rocks expand when heated, and
contract when cooled ; and it has been assumed that when
they pass from a fluid or pasty condition into a solid
crystalline state, the contraction they undergo produces a
considerable diminution of their volume. Hence, it is
said, that if the earth is a cooling globe, it is also a con¬
tracting globe, and that its mass has been continually
shrinking in volume, so that every portion of it has been
compelled to occupy less and less space.
Experiments have been made on various rocks to ascer¬
tain the actual amount of contraction they undergo in
passing from a fluid to a solid state. Bischof experimented
upon granite, trachyte, and basalt, and according to his
results it appeared that granite contracts 25 per cent.,
trachyte 18 per cent., and basalt 10 per cent. Subsequent
investigations, however, have thrown some doubt upon
these results, and those aiiived at by M. Delesse are pro¬
bably nearer the mark. His experiments showed that the
more acid rocks (granite, felsite, etc.) contracted more than
the basic rocks (basalt, etc.), but that the amount of con¬
traction never exceeded 10 per cent., and was more often
' " Physics of the Earth's Crust," pp. 152, 281.
636
PHYSIOGRAPHICAL GEOLOGY. [PAET III.
between 3 and 7 per cent. Mr. Mallet's experiments
indicate a still less amount of contraction in the case
of ordinary slag. Finally, observations on Kilauea seem
to show that solidified basic lava is less dense than
the liquid magma from which it has crystallized, for
crust-like masses sometimes form and float on the top
of the pools in the crater. It may be, therefore, that it is
only the highly silicated rocks which contract on cooling.
We may, however, assume that the continual dissipa¬
tion of heat has caused a certain gradual contraction of
the earth's mass. The materials of the earth conduct
heat very badly ; consequently the surface, being exposed
to the sky, would become covered with a cooled crust,
while at a very moderate depth the temperature was still
very high ; just as a lava stream can be walked over while
it is still molten inside. The circumference of the crust
so formed would be that of the hotter globe. As time
went on, the heat from within would pass out through
the cooled crust, and this would be accompanied by a
shrinking of the interior. But the outside of the crust
was cooled while the globe was larger, and although it will
have grown thicker, still it will remain too large for the
shrunken interior, upon which, therefore, it will settle
•down by its own weight.
It is impossible, however, that there could be any dis¬
severance or break of continuity between the outer and
inner portions of the contracting globe, for no part of the
earth's crust could stand without support from below:
the power of gravitation (or the weight of its own mass)
must crush in the outer layers of the crust and force it to
accommodate itself to the diminishing circumference of the
contracting interior.
It requires only an elementary knowledge of mechanical
principles to perceive that the inward pressure of a rigid
arched envelope or crust must necessarily be accompanied
by enormous lateral pressure. If the interior of the globe
is drawing away from the outer crust, we may look upon
the crust as subjected to vertical forces acting equally
from every direction, and it is clear that these forces will
be accompanied by tangential forces causing lateral pres¬
sure in every portion of the earth's crust.
CHAP. VI.J THK FORMATION OF CONTINENTS. 637
This lateral pressure would produce very powerful
compression and crumpling of the rocks composing
that crust, resulting eventually in their bending, break¬
ing, and ridging up along certain lines. In other
words, the crust could only continue to embrace the
nucleus by a process of crushing and crumpling, and
this process would continue whether the interior were
solid or liquid.
If, therefore, this conception of the mode in which com¬
pression of the crust has proceeded is correct, if the crust
has become wrinkled and contorted in consequence of its
closing in upon the shrinking interior mass of the globe,
then there is no doubt that the pressure thus generated
would be amply sufficient to cause elevations and depres¬
sions of the surface, and might by its repeated action
develop such features as oceans, continents, and mountain
chains, if the globe contracted enough}
Objections to the Theory of Contraction.—Until
quite recently there was a strong disposition among geolo¬
gists to accept the hypothesis of contraction as sufficient
in itself to account for the existence of continents and
mountain chains. Mr. O. Fisher, however, has put the
hypothesis to the proof, and comes to the conclusion that
it is inadequate to account for the production of these
features.' He admits that the great inequalities of the
continental surfaces have been caused by lateral compres¬
sion, but does not believe that this has arisen from the
secular cooling of the earth's mass.
Mr. Fisher considers the hypothesis on two separate
assumptions : first, that the earth is solid, and has cooled
as a solid ; second, that there is a liquid substratum below
the solid crust.
' Mr. Fisher however, has recently shown that the amount of
heat lost by the earth since the first formation of a solid crust may
not have been very great. Assuming the existence of a liquid sub¬
stratum, and that the crust began to be formed one hundred million
years ago, which is an ample allowance for geological time, the
difference between the mean temperature of the interior then and
now would be only 209° F. (Appendix to " Physics of Earth's
Crust," issued separately, 1891, p. 49).
^ " Physics of the Earth's Crust," second edition, p. 121, and
Appendix, p. 29.
638
PHTSIOGRAPHICAL GEOLOGY. [PART III.
1. On tlie first assumption all the inequalities produced
hy compression arising from contraction would be of the
nature of elevations above a certain datum level. There
could be no actual, but only relative, depressions, and the
bottom of the ocean-basins would either coincide with this
datum level or would be the parts that were least raised
above it. Proceeding on this assumption, and taking the
lowest parts of the ocean-floors as a base level, he esti¬
mates that the whole of the existing inequalities of the
earth's surface (above the said datum level), if levelled
down and spread out, would form a uniform layer of about
13,000 feet in thickness.
He then shows that it is possible to estimate the amount
or volume of the inequalities which would be produced by
the contraction of a cooling solid globe. In the first place
it has been shown by Mr. C. Davison and Mr. Mellard
Heade that in such a globe at a comparatively small depth
below the surface (probably within two miles) there would
be, at the present time, a layer or shell, where its material
would be neither compressed nor extended ; this may be
called " the level of no strain." Below it the strata would
be extended, above it they would be compressed. It is,
therefore, entirely out of the part above this level that the
surface elevations could be produced. Mr. Fisher calcu¬
lates what elevations are likely to have been formed by the
compression of this layer, and on the most extravagant sup-
■positions he finds that the mean height of the elevations,
if levelled down into a continuous layer, would be only a
little over 6 feet. Comparing this with the 13,000 feet of
actual elevation, it seems impossible that the latter could
have been formed by the contraction of a cooling solid
globe.
2. The second and more probable case, that there is a
yielding liquid substratum, Mr. Fisher did not fully deal
with in his book, but has done so in his valuable Appendix
(recently issued). He finds that if the age of the world is
100 miUion years, and the co-efficient of contraction for
liquid rock is twice as great as that of solid rock (as
Mallet's experiments seem to indicate), then the depth of
the level of no strain wiU be at present about 4 miles
below the surface, and the average height of the elevations
CHAP. VI.J THE FORMATION OF CONTINENTS.
639
formed by the corrugatioa of the material above that level
•would be about 44 feet. If, however, the age of the world
is taken to be only 50 million of years, the depth of the
level of no strain would be only about 2 miles, and the
average height of the elevations only a little over 10 feet.
These amounts are equally inadequate when compared with
the actual average elevation of 13,000 feet above the datum
level.
It has been suggested, however, by Mr. Mallet and by
Professor Le Conte, that, leaving mountain ranges out of
consideration, the wide continental plateaux and the inter¬
vening oceanic depressions may have resulted from dif¬
ferences in the amount of radial contraction, that is to say,
the crust may have shrunk bodily inwards beneath the
oceanic areas further than it has beneath the continental
areas.
To refute this idea Mr. Fisher has calculated what the
total amount of radial contraction would be. On the
assumption of a solid globe whose outer crust, to a depth
of 402,832 feet (a little over 76 miles), is supposed to have
the conductivity made use of by Sir W. Thomson (and
this limit is taken because below that depth the effects of
cooling would be quite insensible), the contraction comes
out as only 2 miles, with an extreme possibility of 6 miles.
Now the deeper parts of the oceans are between 3 and 4
miles below the average level of the continental plateaux,
and it is impossible that these depressions could be formed
by excess of radial contraction if the total amount of that
was only 2 miles, while if it amounted to 6 miles there is
no reason why the contraction should vary so much in
different places as to cause a difference of 4 miles in
vertical elevation.
In the case of a liquid substratum he finds the radial
contraction may have been greater ; taking values for the
melting temperature and the latent heat of a cooling
liquid magma from experiments recently made by Pro¬
fessors Hücker and Roberts-Austen, and Mallet's result
for the coefficient of contraction, he finds the total amount
will be 6 miles if the age of the world is 50 millions of
years, and 12 miles if its age is 100 millions ; but the
same objection remains as in the former case—-no reason
640
PlIYSIOGRAPHICAL GEOLOGY. [PAET III-
can be assigned for local differences amounting to 4
miles, whatever the total amount of radial contraction
may be.
Captain C. E. Dutton thinks that Mr. Fisher has touched
too briefly upon another argument against the contraction
hypothesis, namely, that the features to be explained are
not such as would have been produced by contraction.^
" The strains set up in the crust by a shrinking nucleus
would be such that for any given amount of corrugation
with the axes (of the folds) in one direction, there must be
an equal amount with the axes at right angles to that
direction. The localization of mountain chains and plica¬
tions in long narrow belts, with the axes of the folds all
approximately parallel, and with no corresponding plica¬
tions at right angles to them, is an impossible result of a
collapsing spherical shell." In other words, if the features
were causèd by contraction, there would be a network of
small mountain chains all over the earth's surface, like the
wrinkles on the skin of a dried apple ; they would not be
nearly so long or so lofty as they are.
Hypothesis of Expansion by Injections of
Lava.—Mr. Fisher formerly thought that the observed
phenomena of compression might be accounted for by the
fissuring of the crust which results in volcanic action, the
fissures opening upwards from the liquid substratum, and
being filled with matter proceeding from it. Looking to the
steam which is always given off during volcanic eruptions,
he believes the liquid magma to contain a large amount of
water-substance (dissolved in the state of gas), and thinks
that all fissures originating on the under surface of the
crust would be immediately filled with this gas, under a
pressure of about 10,066 tons to the square foot ; the gas
would consequently exert this pressure in widening the
fissure, that is to say, in compressing the rocks on either side
of it. If the rent reached the surface, the vapour would rush
forth, and be followed by the magma itself in the form of
lava ; if the rent did not reach the surface, the tension of
the vapour in it would still gradually be diminished, and
the magma would ascend higher and higher, and when
' " American Journ. of Science," vol. xxiii. p. 283.
CHAP. VI.] THE FORMATION OF CONTINENTS.
641
solidified would form one of the dykes of igneous rock
which are so frequent even in the upper part of the crust.
Mr. Fisher still believes that this action supplies a cause of
compression, but observes that " in all physical problems
there is a danger of holding one theory of causation to the
exclusion of others that are compatible with it," ^ and he
thinks the observed amount of compression requires a more
extensive and energetic agency."
Hypothesis of Pressure exerted by Convection
Currents.—This more energetic agency he finds in the
consequences likely to result from the existence of a sub¬
stratum of liquid material cooling by convection. He
points out that the observed movements of the crust causing
upheaval and subsidence of the surface require the condi¬
tions of a thin crust resting in approximate hydrostatic
equilibrium upon a yielding substratum : hence geologists
cannot regard the earth as a solid globe. Further, the
crust formed by cooling out of such a substratum would
by this time have become thick, unless it had been pre¬
vented from thickening by the action of the substratum in
constantly dissolving ofE, and, as it were, washing away its
underside ; not so fast, but nearly as fast, as it solidified.
This, as has been already explained, shows that there must
be convection currents in it (op. cit., p. 356). Such convec¬
tion implies upward and downward currents and resulting
local alterations of temperature.
By making and examining successive assumptions as to
the relative densities of the crust and substratum, both
under the oceans and under the continents, he finds that,
with the conditions that best fulfil the requirements of the
problem, the convection currents are rising beneath the
oceans and descending beneath the continents, so that over
a certain space between them the currents will be hori¬
zontal. This flow of the liquid magma toward the conti-
^ " Physics of the Earth's Crust," second edition, p. 295.
® The repeated transference of material from the interior to the
surface by volcanic action would produce a certain amount of com¬
pression 'by causing the crust to settle down upon a contracted
spherical surface, but the total compression so caused would be
small, and the extravasation of lava is more likely to be a result
of compression than a cause of it.
T T
642
PHYSIOGRAPHICAL GEOLOGY. [PAET III.
nental areas will tend to press the supernatant crust in the
same direction, and to produce compression along the
common boundary of the oceanic and continental areas.
" As soon as compression begins to take effect, roots to the
elevated portions will be produced, which, dipping into the
substratum, will offer an increasing obstacle to the flow
of the currents and intensify their operation" {op. cit.,
p. 376). Owing to the lower parts of the thickened area
being softened by heat, a greater thickness will be sheared
downwards than upwards. Assuming that under lateral
compression two-fifths of the thickened part would go up
and three-fifths would go down, and that the ratio between
the specific gravities of crust and substratum is the same
as that between those of granite and basalt, which is nearly
as that of ice to water, he shows that the thickened tract
Fig. 214. Diagram to show the Roots of a Mountain Chain.
a, Crust, h. Substratum, c. Mountain chain, p. Downward pro¬
tuberance. o o. Ocean level, tn m, Lower mean level of the
earth's crust.
would sag still further downwards.' " Hence depressions
would arise on both sides of the ridge, and the ocean,
which covers the general surface, would be deeper than
elsewhere along two channels parallel to, and at some
little distance from, the ridge. But should the ridge be
steeper on one side than on the other, as seems inevitable,
the ocean would be deeper on the steeper side." This re¬
lation between ocean depths and mountain chains seems
actually to exist, as already mentioned.
Lastly, he considers the consequences of the transference
of sediment from the mountain chains to the oceans, and
Op. cit., pp. 131, 278.
CHAP. VI.J THE FORMATION OF CONTINENTS.
643
shows that on hydrostatic principles the areas from which
matter is removed will rise, while the areas to which sedi¬
ment is transported, being loaded, will sink.
Hence he concludes that the vertical movements affect¬
ing the earth's surface are of two kinds ; one resulting from
compression, the other from the laws of hydrostatic equi¬
librium simply. As the result of the latter, degradation and
elevation are necessarily correlatives of each other ; the
degradation of the tract causes more rock to rise up, to be
in its turn degraded and removed.'
Hypothesis of Cubical Expansion.—Mr. Mellard
Eeade has suggested another cause of compression and ele¬
vation.^ He does not accept the theory of a liquid sub¬
stratum, but considers that at a certain depth the material
of the globe is sufficiently plastic from heat to respond to
changes of pressure. This he calls the shell of greatest
mobihty. He points out that the deposits out of which
mountain ranges have been built are always very thick,
and that they seem to have been laid down in great basins>
or areas of subsidence, that is to say, basins formed by the
bending down of portions of the earth's crust. This bend¬
ing down will locally displace the matter of the shell of
greatest mobility, but at the same time (as previously
shown by Babbage and Herschell) the temperature of the
whole of the depressed crust will be raised.
The rise of temperature so obtained causes expansion of
the rocks, and it is found by experiment that the coefficient
of expansion of average rock is about 2'75 lineal feet per
mile for every 100° F. of rise ; but the expansion is cubical,
and will be thrown into the direction of least resistance.
This Mr. Eeade takes to be upward, and therefore credits
all the cubical expansion {i.e., 8'25 feet per mile) to moun¬
tain building.
There can be little doubt that such cubical expansion
does take place, and that it is an effective cause of com¬
pression, but Mr. Eeade does not sufficiently consider how
it may act and how far it can go. He assumes that, as
soon as the depressed area of crust begins to expand, the
expansion will be localized, and wiU bulge up the rocks in
' Op. cit., p. 224.
' " The Origin of Mountain Ranges," London, 1886.
644
PHYSIOGRAPHICAL GEOLOGY. [PAET III.
the form of a ridge parallel to the longer diameter of the
depressed area. He rests this belief on the results of cer¬
tain experiments on the expansion of metal plates, but as
these plates were under conditions totally different from
those of the earth's crust, they do not afford a sound basis
for his belief.^
Mr. Reade admits that part of the expansion would be
expended in compression and plication, and supplements his
theory by supposing that the first upheaval by expansion is
only the initiation of a mountain range. He thinks that
this will relieve the pressure on the plastic material below,
and that this, becoming liquid, will rise as lava through
cracks in the crust, and by bringing fresh accessions of
heat will cause further recurrent expansions of the rock-
beds below the mountain range.
Summary.—If we now compare the results arrived at
by Mr. Fisher and Mr. M. Reade, who have approached
the same problem from very different points of view, we
find that they agree in one very important point, namely,
that the contraction of a cooling, solid globe will not account
for the geological facts. They differ, however, in their
conception of the condition of the interior, and, conse¬
quently, in their attempts to explain the movements of
the earth's crust. Mr. Fisher refuses to be bound by Sir
W. Thomson's dictum that the globe is a rigid, solid mass ; he
adduces weighty reasons against such a view, and in favour
of the existence of a completely liquid substratum of greater
or less depth ; he believes this liquid magma to be satu¬
rated with gas under high pressure, and he shows how the
movements of such a mobile material may exert a com¬
pressive force on the under side of portions of the crust. Mr.
Reade feels more bound by the theory of effective rigidity,
but he also assumes the existence of a zone of plastic
material which is ready to become liquid on the relief of
pressure ; and he explains crust movements by changes of
volume due to alterations in pressure, and a flow of the
plastic material, the initial cause being sedimentation de¬
pressing a portion of the crust, which expands, and forms a
surface ridge, or moimtain range. To Mr. Reade's theory
^ For a discussion of Mr. Reade's theory and other objections to
it see " Geol. Mag." for January, 1892, p. 24.
CHAP. VI.J THE FORMATION OF CONTINENTS.
645
there are serious objections, while the only grave objection
to Mr. Fisher's is the doubtful argument from the tides ;
and the existence of a liquid substratum is so very pro¬
bable, that it may be accepted as a sound theoretical in¬
ference from the facts which are at present known to us.
VIKW OF THF, ENVIRONS OF CLERMONT. FROM TîlE PUY GlllOU,
rja.u
1
Plaii of úie Cra.ter
Lr. Vacl\e
Pourcharet
MonV -% Dome
LIST OF AUTHOES QUOTED.
a.
Abbaye, Kev. r., 207.
Agassiz, A., 279, 290, 293.
Airy, Sir G. B., 635.
Ansted, Prof., 102.
Ashley, —, 133.
Attfield, Prof., 115.
Aveline, W., 549.
b.
Babbage, C., 252, 266, 643.
Bailey, Prof., 241.
Ball, Dr., 183.
Barker, Dr. J., 242.
Barrois, Dr. C., 534, 536, 592.
Bayfield, Capt., 270.
Beaufort, Sir F., 275.
Beechy, Capt., 103.
Bennett, F. J., 113.
Berger, Dr., 532.
Bird, Miss, 136.
Bischof, G., 114, 132, 276, 325.
Blake, Prof. J. F., 39, 384.
Blanford, W. T., 142, 196, 503.
Boblaye, E. de, 118.
Bonney, Prof. T. G., 104, 127,
130, 230, 233, 343, 406,413, 428,
504, 512,514, 515, 529, 530, 537,
599, 601, 626.
Bourne, J. C., 290.
i Bowerbank, Dr., 398.
Brannon, Mr., 261.
Broeck, Vanden, 89.
Brongniart, a., 68.
Buat, M. du, 130.
Buchanan, J. Y., 183, 273.
Buddle, J., 380.
Bunsen, Prof., 206.
C.
Calloway, Dr., 528, 538.
Campbell, J. F., 103, 611.
Celsius, 66.
Chambers, R., 175.
Cheadle, Dr., 212.
Churchill, 627.
Codrington, T., 594.
Cohn, Dr. F., 205.
Cole, Prof. G., 522.
Conybeare and Phillips, 118.
Coppinger, Dr., 190, 292.
Credner, 627.
Croll, J., 78.
D.
Damour, 207.
Dana, Prof. J. D., 79, 120, 137,
247, 277, 287, 630.
Darwin, Prof. G. H., 16, 50,
183.
648
LIST OF AUTHORS QUOTED.
Darwin, Dr. Ch., 59, 62, 68, 70,
71, 79, 116, 190, 203, 209, 211,
221, 248, 250, 269, 271, 282,
288, 438, 527.
D'Archiac, M., 461.
Daubrée, M. A., 396, 401.
Davies, A. T., 447.
Davison, C., 458, 638.
Dayman, Capt., 297.
Delabeche, H. T., 71, 75, 164,
168, 235, 463.
Delesse, A., 263, 278, 635.
De Ranee, C. E., 185, 222.
Dittmar, Prof., 246.
Drew, F., 223.
Druminond, Prof., 210, 240.
Dutton, C. E., 43, 53, 59, 150,
569, 618, 640.
Du Noyer, G. V., 229.
E.
Ehrenberg, 226, 239, 240.
Erdman, M., 68.
F.
Fergusson, J., 138.
Fielden, Capt., 185.
Fisher, Rev. O., 13, 16, 33, 411,
433, 524, 635, 637, 640, 644.
Fitzroy, Admiral, 58.
Foot, F. J., 404.
Forchhammer, Dr., 186, 247, 271.
Fouqué, M., 17, 47. '
G.
Geikie, Dr. A., 72, 100, 146, 164,
334, 353, 421, 473, 476, 497,
501, 502, 568, 598, 602, 624,
629.
Geikie, Prof. J., 208, 231, 471.
Gilbert and Churchill, 599, 627.
Gilbert, G. K., 100, 243, 414, 492,
627.
Godwin-Austen, 263.
Goodchild, J. G., 274, 278, 401.
Graah, Capt., 74.
Green, Prof. A. H., 48, 143.
Greenwood, Col. G., 140.
Guppy, Dr. H. B., 277, 290, 293.
H.
Hall, Sir J., 433, 508.
Marker, Mr. A., 473, 476, 491,
492, 508.
Harkness, Prof., 18.
Harkness and Blyth, Profs., 521.
Harris, G. F., 406.
Hartley, Sir C. A., 133.
Hartt, 215.
Hatch, Dr., 350.
Haughton, Prof., 485.
Hayden, F. V., 53, 59, 208.
Heim, Prof., 121, 437.
Henslow, Prof., 532.
Henwood, J. W., 11,464.
Hickson, 292.
Hinde, Prof., W. Y., 270.
Hinman, 127, 182.
Holub, E., 117.
Hooker, Sir J., 303.
Hopkins, W., 15, 128.
Howorth, H. H., 200, 623.
Hudleston, W. H., 93.
Hughes, Prof. McK., 88, 121.
Hull, Prof. E., 73, 245, 382, 549.
Humboldt, A. von, 57.
Hunt, A., 265, 358.
Hunt, Prof. Sterry, 133, 274,
396.
Hutchings, W. M., 360, 522.
Huxley, Prof., 298.
LIST OF AUTHORS QUOTED.
649
I.
Irvine and Woodhead, 276.
Irving, Rev. A., 90, 94, 121, 504.
Ives and Newberry, 150.
J.
Johnston-Lavis, 26, 36.
Judd, Prof. J. W., 23, 39, 502,
511.
Jukes, J. B., 1, 3, 46, 79,118, 125,
136, 144, 165, 166, 201, 242,
281, 333, 353, 382, 384, 387,
401, 404, 412, 430, 443, 457,
463, 479, 482, 488, 520, 527,
533, 543, 555, 564, 578.
K.
Kerr, W. C., 192.
Kinahan, G. K., 254.
King, Clarence, 97, 103, 243.
KingsmiU, W., 92.
L.
Lament, J., 162.
Lapworth, Prof. C., 431, 528.
Labour, G., 266, 490.
Leconte, J., 141.
Letheby, Dr., 135.
Lister, J. J., 292.
Livingstone, Dr., 100.
Lockwood, Lt., 231.
LyeU, Sir Charles, 5, 32, 39, 47,
56, 60, 61, 64, 67, 68, 81, 124,
136, 144, 172, 177, 183, 193,
196, 205, 212, 226, 232, 236,
238, 245, 255, 257, 274, 472.
M.
Maccullock, Dr., 491.
Macmahon, Gen., 530.
Mallet, R., 50, 56, 411, 537, 639.
Mantell, Dr. G. A., 35, 71, 123,
260, 377, 467.
Marr, J. E., 508, 577.
Maw, 203, 204.
Medlicott, H. B., 196, 503.
MiaU, L. C., 116.
Miller, Hugh, 222.
Millson, A., 210.
Milne, Prof. J., 186, 271.
Milton, Lord, 212.
Murchison, Sir R., 35.
Murray, J., 274, 276, 290, 294,
300, 634.
N.
Nansen, Dr., 159.
Nares, Sir G., 187.
Nelson, Capt., 214, 285.
Newberry, J. S., 150.
Nordenskiold, Prof., 162, 231.
P.
Page, Dr., 426.
Pengelly, W., 168.
Penning, W. H., 180, 421.
Phillips, Prof. J., 172.
Pickwell, R., 173.
Pingel, Dr., 74.
Playfair, John, 66.
Powell, J. W., 150, 428, 584,
591.
Prestwich, J., 71, 117, 132, 135.
Pumpelly, R., 623.
R.
Ramsay, Sir A., 115, 159, 192,
498, 522, 530, 608, 610, 624,
629, 638, 643.
Reade, Mallard, 77, 132,182, 456,
638, 643.
Redman, J. B., 173, 175.
650
LIST OF AUTHORS QUOTED.
Reid, Clement, 183.
Renard, M., 447.
Rennie, Sir J., 211.
Richardson, Dr., 237.
Riclithofen, Baron, 198, 200, 501.
Risinf», Lecomte and, 464.
Roberts-Austen, Prof., 639.
Roberts, E., 60.
Rosenbuscb, Prof., 536.
Rücker, Prof., 639.
Russell, J. C., 195.
Rutley, F., 334.
S.
Salmon, Mr. Curwen, 443, 479.
Scrope, Poulett, 35, 145, 467, 501.
Sedgwick, Prof. A., 505, 527.
Semper, Prof., 290-292.
Sbipman, J., 549.
Siemens, AV., 17.
Skertcbley, S. B. J., 213, 239,
258.
Smith, AA'., 2.
Solías, Prof. AA'. J., 398.
Sorby, H. C., 324, 351, 357, 387,
522.
Spencer, Prof. J. AA"., 159, 230,
232, 630.
Spratt, Admiral, 47, Capt., 66.
Stallibrass, Mr., 183.
Straban, A., 394.
Stevenson, J., 164.
T.
Teall, J. J. H., 340, 341, 347,
487, 513.
Thomson, Sir AA"., 15, 195, 201,
639.
Tietze, Dr. E., 591.
Tizard, T. H., 256.
Topley, AA"., 426, 441, 459.
Torell, O., 68.
Tornoe, —, 273.
Tylor, A., 128, 141.
Tyndall, 525.
AV.
Webster, T., 175.
Weed, AV. H., 205, 208.
AA'harton, Capt., 45, 291.
Wbitaker, AV., 89, 116, 174.
Willett, H., 17.5.
AA"ilUams, Rev. J., 120.
Wilson, J. M., 432.
AA"ortb, R. N.,478.
INDEX.
A.
ABERGLÄSLYN, Pont, 129.
Abyssinia, plateaux of>
basalt in, 503 ; floods in, 142.
Acid rocks, 339, 344.
Acids, 309.
Actinolite, 321.
Actinozoa, 280.
Adelsberg, cave of, 91,119.
Adria, port of, 225.
Adularia, 318.
Aerial rocks, 332.
Africa, changes of temperature in,
100 ; swallow holes in, 117 ;
work of worms and ants in,
210.
Agate, 314.
Agglomerate, 352.
Aiguilles, 104, 599.
Alabaster, 316.
Albite, 317, 318.
Alkaline waters, solvent power
of, 113.
Alluvial levels, 217, 588 ; fans,
223.
Alps, action of frost in, 104 ;
denudation of the, 618 ; glaciers
of, 15 ; inverted strata of the,
428.
Altenfiord, raised beaches at, 70.
! Alternation of beds, 378.
! America, North, mountains of,
457 ; peat-mosses of, 212 ; "bad
lands " of, 97.
America, South, deposits along
coast of, 250 ; earthquakes in,
58 ; raised beaches of, 70 ;
salinas of, 2^8.
Amethyst, 314.
Amphibole, 320.
Amygdaloid, 339.
Anamesite, 349.
Andalusite, 322 ; -schist, 513.
Andesine, 318.
Andésite, 347.
Anglesey, dyke in, 442 ; denu¬
dation of, 488 ; foliation of, 439.
Anhydrides, 309.
Anhydrite, 367.
Anorthite, 317, 319.
Anthracite, 370.
Anticlinal curves, 423.
Antrim, basalt of, 503 ; dykes in,
489, 532 ; unconformity in, 559;
volcanic rocks of, 469.
Ants, soil formed by, 210.
Apatite, 317.
Aphanite, 348, 350.
Appalachian Mountains, fault in,
611.
652
INDEX.
Aral, Lake, 244.
Ardennes, metamorphism in,
447.
Arenig Hills, 575.
Argillaceous rocks, 360.
Argillite, 507.
Arkose, 358.
Arragonite, 316.
Artesian wells, 110.
Arthur's Seat, 500.
Asbestos, 321.
Ascension, Isle of, 277.
.A.sli, volcanic, 26, 352.
Asia Minor, deposits on coast of,
275.
Asphalt, 370.
Astrakanite, 244.
Athabasca, deposits in Lake,
237.
Atiu, caves of, 120.
Atlantic, depth of, 297, 633 ;
ooze of the, 297 ; red clay of,
299.
Atmospheric agencies, 84, 105,
583.
Atolls, 282.
Augen-structure, 506.
Augite, 320.
Aureoles, 535.
Australia, blown sands of, 201 ;
coral-reefs of, 281 ; mountains
of, 633 ; soundings off north
coast of, 286.
Auvergne, basalt of, 406 ; valleys
of, 145 ; volcanoes of 467.
Axmouth, landslip near, 123.
Ayrshire, volcanic necks of, 471.
B.
" Bad lands " of North America,
97.
Bahama Islands, chalky mud of,
286 ; coral rock of, 285 ; man¬
groves of, 214.
Baiœ, bay of, 62.
Baltic, diatoms in the, 303 ; ice
in, 186, 271 ; rise of its shores,
66.
Banded gneiss, 530.
Barbados, caverns in, 120 ; coral-
rock of, 288 ; faults and folds in,
456 ; radiolarian earth of, 302 ;
red clay of, 195 ; tertiary chalk
from, 363.
Bargate stone, 367.
Barnwell, gravel of, 220.
Barren island, 24.
Barrier reefs, 281.
Barrow Hill, 473.
Basalt, 349.
Basaltic dykes, 532.
Basaltic plateaux, 501.
Bases, 309.
Base-line of erosion, 585.
Basic rocks, 339, 344.
Basset edges, 422.
Bath waters, temperature of, 13 ;
minerals dissolved in, 115.
Batt, 361, 382.
Beachy Head, 174.
Beavers, work of, 212.
Berg-mehl, 239.
Bermudas, red soil of, 195 ; sound¬
ings off, 286.
Bessarabia, limans of, 248.
Bilin, siliceous earth of, 240.
Bind, 361.
Biotite, 320.
Bitter lakes, salt in, 247.
Bitumen, 370.
Bituminous limestone, 366.
Blanc, Mont, section through,
431.
Black earth of Russia, 198.
INDEX.
653
Blackwater river, 578.
Bloodstone, 314.
Blown sand, 200.
Blue muds, 294.
Bog-iron-ore, 241.
Bog-oak, 211.
Bosses of igneous rock, 472.
Boulder clay, 362 ; formation of,
230.
Boulders, dispersal of, 154.
Bradford clay, 375.
Breccias, 191.
Brighton, raised beach at, 71.
Brimham rocks, 99.
Bristol coalfield, 553.
Bronzite, 321.
Burford, coal-measures under,
560.
Buried valleys, 76.
Burntisland, 99 ; volcanic rocks
of, 533.
Burren, joints in limstone of,
404.
Buttes, 603.
C.
Calabria, earthquakes in, 55.
Calcareous deposits, 238, 273.
Calcareous mud and limestone
paste, 277.
Calcareous rocks, 362 ; conversion
of, into ironstone, 93 ; weather¬
ing of, 86.
Calcite, 316.
Calcic sulphate. See Gypsum.
Calc-schist, 514 ; varieties of, 362.
Calcutta, well at, 226.
California, mineral veins in, 464.
Cambridge Greensand, 551.
Cambridgeshire, peat of, 213 ;
river gravels in, 220 ; shell
marl in, 239.
Canada, peat-mosses of, 212.
Cannel-coal, 369.
Cañons, formation of, 148, 590.
Cantabria, elevation of, 592.
Canton, granite of, 92.
Cantyre, raised beaches of, 73.
Caraccas, earthquakes at, 49.
Carbonate of lime, 316 ; deposi¬
tion of, 205, 242 ; in rivers,
131 ; in the sea, 273 ; solution
of, 86.
•Carbonate of iron, 317.
Carbonic acid, action of, 85 ; in
the sea, 273, 274.
Carlsbad, springs of, 114.
Carnarvonshire, volcanic rocks
of, 473.
Carnelian, 314.
Carolina, North, 192.
Caspian Sea, salts in, 244.
Cat's-eye, 314.'
Cave-earth, 197.
Caverns, origin of, 115.
Caves, sea, 73 ; formation of, 166.
Chalcedony, 314.
Chalk, 304, 363.
Chalybite, 369.
Chanonat, valley of, 145.
Chara, 238.
Charleston, earthquake of, 53, 59.
Chemical action of rain, 84 ; de¬
posits, 202, 208,241 ; elements,
308.
Chert, 314, 359, 398.
Chesil bank, 254.
Chiastolite, 322, 513.
Chili, earthquakes in, 58.
China, loess of, 198.
Chlorite, 323 -, -schist, 513,
Chrysolite, 321.
Cinerite, 28.
Cirques, 600.
654
INDEX.
Clapham Cave, 120.
Clay, 359 ; with flints, 193.
Clay-ironstone, 398.
Cleavage, of crystals, 310 ; of
rocks, 505, 517 ; production of,
524.
Cleveland Dyke, 410.
Cliffs, waste of, 168 ; formation
of, 170.
Clinometer, 417.
Clunch, 364.
Coal, 369 ; joints in, 402 ; seams
of, 382.
Coal-measures, 379, 383, 429, 443,
559.
Coast-ice, 184, 268.
Cols, formation of, 602.
Colorado, canons of, 148, 590 ;
plateaux of, 427.
Columnar structure, 409.
Compression, proofs of, 521 ; con¬
tortions produced by, 430 ; table
of, 432.
Concretions, 396.
Conformable succession, 543.
Conglomerate, 356.
Consolidation of rocks, 387.
Contemporaneous erosion, 558.
Continents, formation of, 631.
Contortions, 430.
Contraction, faults produced by,
456 ; joints produced by, 400 ;
theory of secular, 635.
Convection currents in liquid sub¬
stratum, 641.
Coprolites, 398.
Coquimbo, raised beaches at, 71.
Coral mud, 286.
Coral reefs, growth of, 79, 280,
288.
Coral rock, 284, 287.
Cordierite-gueiss, 514.
Cork, valleys of, 575.
Cornstone, 358, 397.
Cornwall, coast of, 166 ; elvans
of, 351, 488 ; granite of, 93 ;
mines of, 12; raised beaches
of, 71 ; sand-drifts of, 200 ; sub¬
merged forests of, 75.
Corrasion, 107.
Carries, 600.
Coseguina, 28.
Crag, decalcification of, 89.
Crenic acid, 85.
Cretaceous rocks, overlap of, 555.
Crete, changes of level in, 66,
74.
Crich Hill, map of, 614.
Crinoidal limestone, .363.
Cromer;, loss of land at, 173.
Crush-breccia, 356.
Crust of the earth, 15.
Crystallography, 310.
Cubical expansion, theory of,
643.
Current bedding, 384.
Current-mark, 386.
Currents, action of, 181.
Curvilinear bedding, 385.
Curvitabular joints, 406.
Cyanite, 322.
D.
Dacite, 345.
Dalmeny, dolerite at, 472.
Danube, matter transported by,
135.
Dartmoor, granite of, 477.
Dead Sea, salts in, 244.
Decalcified sand, 89, 90.
Decomposition of rocks, 86.
Deep Sea deposits, 294.
Deltas, formation of, 224 ; sub¬
marine, 266.
INDEX.
655
Denudation, definition of, 106 ;
proofs of, 607 ; results of, in
South Wales, 608.
Deposition of detritus, 216, 234 ;
increased by depression, 261 ;
limits of, 266 ; of salts in solu¬
tion, 176, 203.
Deposits, cavern, 203; chemically
formed, 202, 241 ; deep-sea,
294 ; fluviatile, 216 ; in shallow
seas, 250, 262 ; lacustrine, 234 ;
of organic débris, 278 ; terres¬
trial, 181.
Depre.ssion of land, proofs of, 74 ;
effects of, on erosion, 105, 587,
593.
Depth and thickness, measure¬
ment of, 421 ; table, 423.
Derivative rocks, 355.
Desiccation, 387.
Detrition, definition of, 107 ; re¬
sults of, 104 ; proofs of, 607.
Devil's Cheese Ring, 101.
Devitrification, 338.
Devon, coasts of, 166 ; cleavage
in, 523 ; granite of, 93, 420.
Diabase, 490, 510.
DiaUage, 321.
Diatom earth, 239 ; ooze, 303.
Diopside, 320.
Diorite, 347.
Dip, 416, 516 ; of cleavage planes,
433.
Dip slope, 570.
Disintegration, processes of, 83.
Dislocations, 435.
Dogger Bank, 93, 263.
Doggers, 397.
Dolerite, 349, 408, 510.
Dolomite peaks, in decay, 105 ;
formation of, 287, 365.
Dore, Mount, 467.
Dorsetshire, coast of, 167, 171 ;
overlap in, 535.
Drainage, basins of, 483.
Drei Zinnen, 599.
Dublin county, overlap in, 556.
Dunes, 200.
Dunite, 350.
Dunwich, waste of land at, 173.
Durdle Cove, view of, 169.
Dust-storms, 197, 200.
Dykes, 21, 487, 532.
E.
Earth, crust of, 15, 582; internal
condition of, 14 ; internal heat
of, 10 ; shape of, 8 ; surface
sculpturing of, 564.
Earth-movements, influence of, on
erosion, 582.
Earth pillars, 95, 96.
Earthquakes, 49 ; origin of, 51 ;
phenomena of, 54 ; surface
eflfects of, 61.
Eastbourne, loss of land near,
175.
Egypt, blown sand in, 99, 201.
Eifel, extinct volcanoes of, 472 ;
valleys of, 147.
Elevation of land, 65 ; influence
of, on erosion, 583, 585.
Elvanite, 345.
Elvans, 487.
Encrinites, 375.
English Channel, deposits in,
263.
Enstatite, 321.
Epidiorite, 510.
Epidote, 323.
Episodes, 383.
Erosion, 136, 567 ; base level of,
585; marine, 171, 592; modi¬
fied by earth-movements, 583,
666
INDEX.
584; by rivers, 136 ; of valleys,
140, 586.
Erratics, 154.
Escarpments, breaching of, 578 ;
formation of, 569 ; recession of,
571.
Etna, description of, 38 ; lava-
streams of, 31.
Eulysite, 350.
Euphotide, 510.
Eurite, 344.
Expansion, theory, 640, 643.
F.
Falcon Island, 45.
Falkland Islands, 191, 250.
False-bedding, 385.
Fan-structure, 431.
Fans of alluvium, 223.
Faults, 435 ; age of, 452 ; com¬
pound, 440 ; and folds, connec¬
tion between, 459 ; magnitude
of, 450, 611 ; open and close,
437 ; origin of, 455 ; reversed,
453 ; strike, 448 ; transverse,
444 ; trough, 441.
Felsite, 345, 346.
Felspars, 317.
Felspathic rocks, weathering of,
91.
Felstone, 350.
Felstone-tuff, 352.
Fenland, peat of the, 211 ; forma¬
tion of, 213.
Ferruginous rocks, 367 ; weather¬
ing of, 89.
Fifeshire, volcanic rocks of, 470,
500, 533.
Finsterbach, earth pillars of, 95.
Fiords, 78.
Fire-clay, 360.
Firehole River, 208.
Fire-stone, 359.
Fissures, 31, 56.
Flagstone, 358.
Flaser-structure, 506.
Flexures, 423 ; cause of, 431 ; in
connection with faults, 444,456.
Flint, 314, 359, 398.
Floods, results of, 142.
Fluor spar, 316.
Fluviatile agencies, 126, 584 ;
deposits, 216.
Foel-Fras, 475.
Foliation, 505, 527.
Foliated rocks, 511.
Foraminifera, 294, 324.
Franconia, caves of, 119.
Freagh Hill, section through, 552.
Frost, action of, 102.
Fullers' earth, 361.
G.
Gabbro, 348, 510.
Gaize, 359.
Galionella, 241.
Galliard, 359.
Galway, granite of, 448.
Ganges, delta of, 226.
Gangue, 461.
Gannister, 361.
Garnet, 323.
Gases, dissolved, 17, 33.
Gault, erosion of, 551.
Gaylenreuth, cave of, 119.
Genetic classification of rocks,
330.
Geneva, lake of, 236.
Geognosy, 5.
Geology, definition of, 1 ; Dyna¬
mical, 7 ; Physiographical, 452;
Structural, 303.
Geological map and section, 418.
Geysers, 13, 206.
INDEX.
657
Giants' Causeway, 409, 503.
Gibraltar, breccia of, 192.
Glaciers, work of, 151, 267.
Glauconite, 295, 324, 367 ; sand,
weathering of, 90.
Globigerina ooze, 296.
Gloucestershire, unconformity in,
553.
Gneiss, 514, 530 ; Archaean, 540.
Graham's Island, 592 ; view of, 45.
Graian Alps, view in the, 600.
Grangemouth, buried valley at,
78.
Granite, 344 ; decomposition of,
91, 106, 483 ; metamorphic,
534 ; of Cornwall, 477 : of Dart¬
moor, 477 ; of Leinster, 481,
534 ; position of, 479 ; tongues
and veins, 484.
Granulite, 512.
Granulitic structure, 339, 506.
Graphic granite, 344.
Graphite, 313.
Gravel, deposited by rivers, 217 ;
ridges capped by, 604 ; terraces
of, 219, 220.
Greece, underground channels in,
118.
Green marls, 295.
Green River, 148, 605.
Greenland, coast-ice of, 169, 185 ;
glaciers of, 161, 268 ; subsi¬
dence of western coast, 74.
Greensand, 295, 324, 359.
Greenstone, 350.
Greenstone tuff, 353.
Greissen, 510.
Grenelle, artesian well at, 12.
Greystone, 357.
Grey wethers, 98.
Grinnell Land, 185, 187, 232.
Griqualand, swallow holes in, 117.
Gritstone, 357.
Groups and Episodes, 383.
Gypsum, 316, 367; deposits of,
206.
H.
Hade of faults, 435.
Haematite, 315, 368.
Halleflinta, 507.
Hampshire, erosion of coast in,
177.
Hauyne, 323.
Hawaii, volcanoes of, 22, 25, 40.
Head, formation of, 193.
Headlands, origin of, 187.
Heat, and cold, action of, 100 ;
and pressure, 324.
Heaves, 447.
Heligoland, erosion of, 177, 180.
Hertfordshire pudding-stone, 394.
JEighlands, metamorphism of the,
538.
HiUs and valleys, origin of, 619.
Himalayas, 634 ; breaching of, by
R. Indus, 591 ; roots of the, 635.
Himley, fault at, 443.
Hitchin, gravel pipes at, 116.
Holland, the delta of the Rhine,
225.
Holyhead, submerged forest at,
76.
Hornblende, 320 ; gneiss, 514 ;
schist, 513.
Humber estuary, 258.
Humic acids, 85.
Hunstanton, submerged forest at,
76.
Hurst Castle bank, 255.
Hyalite, 314.
Hydrates, 309.
Hydraulic limestone, 364.
Hypersthene, 321.
D" V
658
INDEX.
I.
Ice, on land, 151 ; on the sea,
184, 267.
Icebergs, formation of, 267.
Icefoot, 184.
Iceland, g> psum in, 206 ; siliceous
sinter in, 206.
Idaho, lava plains of, 501.
Idocrase, 323.
Igneous rocks, 330, 336 ; chemical
composition of, 339 ; classifica¬
tion of, 342 ; mineral composi¬
tion of, 340 ; regarded as rock-
masses, 465 ; specific gravity of,
341 ; structure of, 337.
Ilacolumite, 392.
Ilfracombe, cleavage near, 523 ;
inversion near, 345.
Ilmenite, 315.
Inclination of beds, 416.
Incrustations, 202.
India, floods in, 142 ; sand drift
in, 201 ; basalt plateaux of,
407.
Indian Ocean, depth of, 634.
Indus, gorge of the, 223, 482.
Infiltration, 392.
Inliers, 611.
Interbedding, 378.
Interbedded lavas, 494, 497.
Intrusive sheets, 490.
Inversion of beds, 429.
Ireland, Central, subterranean
streams of, lis.
Ireland, South, contortions in
rocks of, 434 ; detrition of,
610 ; bills and vaUeys in, 477 ;
formations in, 404.
Ireland, West, marine erosion of,
166 ; landslips in, 124 ; volcanic
rocks of, 409.
Irish Sea, deposits in, 265.
Ironstone, 93, 368.
Iron pyrites, 315.
Irwell valley, gravels of, 222.
Ischia, volcanoes of, 37.
Isoclinal curves, 428.
J.
Jamaica, red soil of, 194.
Jarvis Island, gj'psum in, 247.
Jasper, 314.
Java, gullies in, 137.
Joints, 400 ; cuboidal, 402 ; sur¬
face exhibition of. 403 ; hori¬
zontal, 405 ; prismatic, 407.
Jupiter Serapis, temple of, 63.
K.
Kaibab plateau, structure of, 427.
Kalkhomfels, 508.
Kanturk, contorted coal-measures
at, 431.
Kaolin, 91, 195, 197, 359.
Karren-felder, 88, 194.
Katavothra, 118, 194, 197.
Kent, coast-5 of, 174.
Kentish rag. 367.
Kentish Town, well at, 13.
Kersantite, 348.
Kilauea, 41, 43, 636.
Killarney, moraines near, 229.
KiUas, 534.
Killeen, cleavage at, 521.
Killiney, granite areas at, 485.
Kilsith, buried valley at, 78.
Krakatau, 28.
L.
Labrador, ice on coast of, 270.
Labradoiite, 318.
Laecolites, 492.
INDEX.
659
Lakes, deposits in, 234 ; origin
of, 159, 621.
Lamination, 372.
Land, depression and elevation
of, 65.
Land-sculpture, 564.
Landslips, 121, 627.
Lapilli, 26.
Laterite, 196.
Lava, filling fissures, 21 ; inter-
bedded sheets of, 493 ; floods
of, in Western America, 405 ;
streams of, 29, 495 ; submarine,
44.
Leinster, foliation in, 534 ; granite
of, 481.
Lepidolite, 320.
Lepidomelane, 320.
Leucite, 319 ; dolerite, 349.
Lewes levels, 213, 260.
Lherzolite, 349.
Lias, contemporaneous erosion
of, 550.
Lignite, 237, 369.
Limerick, volcanic rocks of, 498.
Limestone, altered, 507 ; defini¬
tion of, 362 ; formation of, 250,
275, 277 ; hiUs, 559 ; solution
of, 86, 625 ; varieties of, 362.
Limestone banks, modern, 279.
Limonite, 241, 315.
Lincolnshire, action of rain in,
98 ; gain of land in, 258 ; es¬
carpments of, 572.
Lipari Islands, 37, 39.
Liparite, 345.
Liquid substratum, 17, 33, 638.
Lisbon, earthquake of, 55.'
Lithographic stone, 364.
Littoral deposits, 255.
Lochlands, 236.
Lodes, 461.
Loess, 197.
London, ancient rocks below, 560.
Luxullianite, 510.
Lydian-stone, 507.
M.
Macclesfield Bank, 291.
Mackenzie River, 237.
Maelar, Lake, 66, 68.
Maelstrom, 181.
Magnetite, 315, 369.
Magnesian limestone, 365.
Malmstone, 359.
Mammoth Cave of Kentucky,
120.
Manganese nodules, 300.
Mangroves, 214.
Map, construction of geological,
418.
Marble, 364, 507.
Marcasite, 31.5 ; nodules of, 398,
399.
Margarodite, 320.
Marine agencies, 165 ; compared
with subaerial, 595 ; deposits,
249 ; deposits due to action of
ice, 267.
Marineorganisms, chemical action
of, 276.
Marl, 361, 364.
Marlstone, .364, 368, 550.
Marocco, calcareous tufa in, 203.
Marshland of Lincolnshire, 258.
Mauna Loa, eruption of, 42.
Mechanical action of rain, 94.
Mechanical deposits, 234, 251,
256, 262.
Meerschaum, 325.
Melaphyre, 350, 510.
Mendip Hills, rock removed from,
610.
Menelite, 314.
660
INDEX.
Mersey, buried valley of the, 77.
Mesas, 603.
Metacrasis, 504.
Metamorphism, 329 ; local, 531 ;
district and regional, 536.
Metamorphic rocks, 329, 331,
332, 504, 5.17.
Metatropy, 504.
Metaxis, 504.
Metis island, 46.
Meyringen, 143.
Mica, 319.
Mica-traps, 347 ; -schists, 512.
Microcline, 317, 318.
Micrographic structure, 339.
Microliths, 339.
Microscope, use of the, 333.
Milledgeville, ravine at, 136.
Minchinhampton, curvilinear bed¬
ding near, 385.
Minerals, rock-forming, 313.
Mineral veins, 461.
Mines, temperature of deep, 11.
Minette, 347.
Mississippi, delta of, 226 ; matter
transported by, 135.
Monkwearmouth, mine at, 12.
Monoclinal curves, 427, 460.
Monoliths, 408.
Moraines, 154, 228, 628.
Morecambe Bay, 256.
Moselle, valley of the, 147.
Mould, formation of, 209.
Mountain chains, age of, 626 ;
height of, 634 ; origin of, 455 ;
roots of, 642 ; as proofs of de¬
trition, 642 ; peaks, 597.
Mourne mountains, granite of,
482.
Mud-flats, 257.
Muds, deep sea, 294.
Mudstone, 358.
Mundesley, waste of clift's at, 173.
Muscovite, 319.
Mylonite, 506, 512.
Mynydd Mawr, 475, 498, 575.
N.
Nailbournes, 112.
Necks, volcanic, 468.
Needles, view of the, 170.
Nepheline, 319.
Nephelinite, 349.
Nevada, U.S., salt field in, 243.
Newfoundland, coast-ice at, 272.
New Orleans, well at, 226.
New Zealand, Alps, 152 ; earth¬
quakes in, 60 ; geysers of, 207.
Niagara, 138.
Nile, delta of, 227 ; matter trans¬
ported by the, 135.
Nodules, concretionary, 396.
Norfolk, fens of, 213 ; loss of
land along coast of, 172, 183 ;
sand hills of, 200.
Norite, 348.
Northampton ironstone, 93.
Northampton Sands, erosion of,
551.
North Sea, deposits of, 262.
Norway, elevation of, 70.
Nosean, 323.
Nottingham, contemporaneous
erosion near, 549.
Nullipores, fresh-water, 242 ;
marine, 284.
Nummulite limestone, 363.
0.
Oahu, beach of, 277 ; coral rock
of, 288.
Oblique bedding, 384.
Obsidian, 346, 483.
INDEX.
661
Oceanic deposits, 244.
Oceans, depth of, 633.
Ochil Hills, 501.
Oil-shale, 371.
Old Red Sandstone, 450,501, 552.
Oligoclase, 318.
Olivine, 322.
Onyx, 314.
Ooiite, 363 ; formation of, 285.
Ooze, globigerina, 296 ; radio-
larian, 302 ; diatom, 303 ; ptero-
pod, 295.
Opal, 314.
Ophicalcite, 508.
Ophitic structure, 338.
Organic deposits, 209, 237, 278.
Orthoclase, 92, 318.
Ortho-felsite, 346.
Outcrop, 417, 607.
Outliers, 611.
Overlap, 547.
Overstep, 548.
Oxide of iron, 314.
Oxides, 309.
Oxygen, action of, 308.
P.
Pacific Ocean, 633 ; deposits and
depth of, 299.
Pampas of S. America, 620.
Pan-ice, 270.
Paragenesis, of minerals, 341.
Paris, artesian wells near, 13.
Passes, formation of, 602.
Patagonia, deposits off coasts of,
190, 250 ; terraces in, 221.
Peat, formatioh of, 211.
Pedestal-boulders, 88.
Pegmatite, 339, 344.
Pennine fault, 451.
Pentland Hills, 501.
Peperino, 28, 352.
Periclinal curves, 426.
Peridote, 321.
Perthite, 318.
Petrifaction, 325.
Petroleum, 370.
Petrology, 307.
Pevensey levels, 213, 260.
Phacoids, 506.
Phonolite, 319, 347.
Phosphate nodules, 317, 398.
Phyllite, 509.
Physiographical Geology, 561.
Picrite, 350.
Pipe-clay, 360.
Pisolite, 364.
Pitchstone, 346.
Plains of construction, 619 ; of
erosion, 565, 619.
Plagioclase felspars, 318.
Pianation, 565.
Plombières, silicates of, 396.
Plutonic rocks, 337, 465, 483.
Po, delta of the, 225.
Polycistina, 301.
Pompeii, destruction of, 36.
Poole Harbour, silting up of, 261.
Porcellanite, 507.
Porphyrite, 339.
Porphyritic structure, 338.
Porphyroid, 514.
Porphyry, 339.
Portland, quarry at, 375.
Port Valíais, 236.
Pressure, results of, 391.
Procida, 37.
Propylite, 511.
Protogine, 514.
Pseudomorphs, 325.
Pteropod ooze, 295.
Pumice, 28, 300.
Pumiceous sand, 353.
Purbeck, isle of, 177.
U tJ 2
662
INDEX.
Puy de Dome, 467.
Pyroxene, 320.
Q.
Quarrying, art of, 414.
Quartz, 314.
Quartz felsite, 345, 483 ; trachyte,
345 ; porphyrite, 345 ; schist,
513.
Quartzite, 507 ; hills, 597.
Queensferry, joints at, 408.
Quiriquina, earthquake at, 59.
Quito, earthquake at, 57.
R.
Radiolarian ooze, 301.
Rain, chemical action of, 84; me¬
chanical action of, 94, 98.
Rain-gullies, 136.
Rain-wash, 189.
Raised beaches, 68, 70-72.
Ravenspur, destruction of, 173.
Reculver, loss of land at, 174.
Red clay of Atlantic, 299.
Red earth, 194.
Reversed faults, 453.
Rhine, country near, 498 ; delta
of, 225.
Rhone, delta of, 225, 236 ; matter
carried in solution by, 134.
Rhyolite, 345, 483.
Richmond, diatom earth of, 304.
Rimutaka Hüls, upheaval of, 60.
Riobamba, earthquake at, 58.
Ripple drift, 386.
Rivers, action of, 126 ; breach of
escarpments by, 578 ; deposits
formed by, 216, 251 ; erosion
by, 136 ; how formed, 126 ;
salts dissolved in, 131 ; trans¬
portation of matter by, 128;
velocity of, 127.
Roach, 368.
Roches moutonnées, 158
Rock, geological meaning of
word, 326; rock-hasins, 629;
rock-.slices, preparation of, 334.
Rocks, analysis of, 334 ; classifi¬
cation of, 326 ; fragmentary,
328 ; cry.stalline, 327 ; igneous
or original, 331, 336, 463 ; deri¬
vative or stratified, 331, 355 ;
metamorphic, 332, 506, 517 ;
vitreous, 337.
Rock-salt, 315, 365 ; formation
of, 244, 247 ; solubility of, 625.
Rocky Mountains, action of frost
in, 103, 192 ; detrition of, 618.
Romney Marsh, 260.
Rose Bridge Colliery, 12, 395.
Rossberg, landslip of the, 122.
Rossthwaite, old lake at, 236.
Rotomahana, geysers at, 207.
Rottenstone, 89.
Rowley Regis, basalt of, 413,
533.
Russia, plains of, 632.
S.
Sahara Desert, 632.
Sahlite, 320.
Salt. See Rock-salt.
Salt lakes, 242.
Salts, 309 ; in springs, 114 ; in
lakes, 243 ; in the sea, 246, 276 ;
deposition of, 243.
Sand, blown, 99 ; formation of,
130.
Sand-dunes, 200.
Sand-flats and hanks, 255.
Sandstone, 357, 397 ; -hills, 598.
Sandwich Islands, 277.
Sanidine, 318.
San FUippe, travertine at, 205.
INDEX.
663
Santa Maria, elevation of, 58.
Santorin, 17, 47.
Sarsen-stones, 98.
Satin-spar, 316.
Saussurite, 510.
Scandinavia, elevation of, 69.
Scania, subsidence of, 75.
Schalstein, 514.
Schists, 511, 528, 534.
Schorl, 323 ; -rock, 510.
Scoriae, 26.
Screes, formation of, 191.
Sea, action of, 164; deposits in the,
249, 273 ; sea-ice, 184, 267; sea-
water, composition of, 246 ; sea-
wave, 54. See Marine.
Sections, artificial and natural,
418.
Secular contraction, 6.35.
Sediment, deposition of, 224, 249,
252.
Segregation veins, 485.
Seismometers, 52.
Selenite, 316.
Selsey Bill, gain of land at, 261 ;
loss of land at, 175.
Septaria, 397.
Serpentine, 322, 354, 511 ; schist,
513.
Sericite, 320; -schist, 513.
Shale, 361, 381.
Shallow water deposits, 250.
Shap granite, 508.
Shear surfaces, 524.
Sheffield, flood near, 143.
Shell marl, 238.
Shell sands, 278.
Sheppey, landslips in, 121, 174.
Shingle beaches, 253.
Shore platforms, 171, 593.
Sierra Nevada, of America, 104 ;
of Spain, 102.
Silica, crystallized, 314 ; dissolved
in river water, 133 ; as flint
and chert, 359.
Silicates, decomposition of, 86.
Siliceous deposits, 239, 301.
Siliceous sinter, 206.
SiUs, 490.
Silt and mud flats, 256.
Silting up of bays, etc., 259.
Simeto, gorge of the, 144.
Skaptar Yokul, eruption of, 31,
32.
Skye, dykes in, 491.
Slate, 509.
Slickensides, 438.
Slievenamuck, fault at, 450.
Smaragdite, 510.
Snow, its conversion into ice,
151.
Snowdon, 497, 519.
Soapstone, 325.
Sodium chloride, deposition of,
243.
Soil, formation of, 189, 209.
Solfatara, lake of, 244 ; crater of,
64.
Solution of rock, 115; lakes
formed by, 627.
Somersetshire, coal-field of, 559.
Spain, elevation of north coast,
592.
Specific gravity of rocks, 341.
Sperenberg, boring at, 13, 395.
Spheroidal structure, 412.
Spitzbergen, 103, 162.
Spotted schist, 513.
Springs, causes of, 108 ; deposits
from, 204 ; hot, 13 ; intermit¬
tent, 111 ; minerals dissolved
in, 204.
Sprudelstein, 114, 205.
Spurn Head, 172.
664
IÑDEX.
StaíFa, Isle of, 409.
Stalactite and stalagmite, 203,
316.
Staurolite, 322.
Steatite, 325.
Step-faults, 441.
Stockholm, elevation near, 66.
Stone rivers, 191.
St. Paul's Island, 46.
Stratification, 373 ; curvilinear,
3S,") ; diagonal, 385.
Stratified rocks, 303, 329, 332 ;
their origin, 355.
Strike faults, 448.
Strike of beds, 417.
Stromboli, 33, 37.
.St. \'incent, 49.
Subaerial agencies, 595.
Submarine deltas, 266 ; platforms,
.794.
Submerged forests, 75.
Subsidence. See Depression.
Subterranean streams, 117.
Suez, Isthmus of, 247.
Sutlolk, erosion of coast of, 172.
Sulphate of lime. See Gypsum.
Sulphates, formation of, 25.
Sulphur Bank, mineral veins at,
464.
Sulphur formed near volcanoes,
2.-).
Superior, Lake, 236.
Surface agencies, 80.
Sussex, erosion of coast of, 174 ;
marshes of, 260.
Sutherland, structure of, 538.
Swallow holes, 115, 625.
Swamp, Great Dismal, 212 ; man¬
grove swamps, 214.
Sweden, changes of level in, 66 ;
raised beaches of, 68 ; subsi¬
dence in South, 75.
Switzerland, action of frost in,
104 ; contorted rocks in, 428 ;
landslips in, 121.
Syene, monolith at, 408.
Syenite, 346.
Synclinal curves, 423.
T.
Table of depth and thickness,
423.
Table-lands, formation of, 592.
Tachylite, 349.
Talc, 319-324 ; -schist, 513.
Tarns, origin of, 624.
Tectonic classification of rocks,
329.
Tees, valley of the, 77.
Temperature in deep mines, 11 ;
effects of rapid changes in, 100.
Teneriffe, Lsland of, 24.
Tephrite, 348.
Terraces, marine, 73, 207 ; of
river-gravel, 219 ; cut out of
silted-up valleys, 221.
Terrestrial agencies, 82 ; deposits,
181 ; temperature, 100.
Terrigenous deposits, limits of,
266.
Thames, matter transported by,
136 ; salts dissolved in, 132 ;
silica dissolved in, 133.
Thanet, Isle of, 259.
Theralite, 348.
Therasia, island of, 47.
Thermal springs, 114.
Thick-coal of Staffordshire, 382.
Thickness of beds, 380 ; measure¬
ment of, 421 ; table of depth
and, 423.
Thickness of reef-rock, 287.
Thinning out of beds, 380.
Thomastown, section near, 553.
INDEX.
665
Throw of faults, 436, 442.
Thrust-planes, 453.
Tiburstone, 204, 245.
Tidal currents, action of, 181.
Tierra del Fuego, 211.
TÜ1, 230.
Tinnevelly, blown sand at, 201.
Tipperary, fault in, 450.
Tivoli, flood at, 143 ; travertine
at, 245.
Tonga Islands, 292.
Tonalité, 345.
Tourmaline, 323 ; granite, 510.
Trachyte, 346.
Transferred drainage, 576.
Transgression, 548.
Transvaal, swallow holes in the,
117.
Trap, 336.
Trappean rocks, 336.
Travertine, 204, 245.
Tremolite, 321.
Tripoli-stone, 241.
Troctolite, 349.
Tufa, 204, 245.
Tuft', volcanic, 28.
Tundras of Siberia, 620.
Tunguragua, 58.
Tuscany, travertine in, 204.
Twin crystals, 313.
Tyne, vaUey of the, 77.
Tyrol, earth pillars of the, 95.
Ü.
Uddevalla, shells at, 68.
Uinta Mountains, 104, 596, 611 ;
scenery of, 568.
Unconformity, 542.
Underclay, 379.
Undercliif, the, 123.
Underground circulation of water,
108.
Underlie of faults, 437.
Unstratified rocks, 329.
Upernivick Glacier, 160.
Upheaval. See Elevation.
Uralite, 321.
Utah, bad lands of, 97, 100 ; salt
lake of, 243.
V.
■yal del Bove, 38, 321.
Valentía, cleavage at, 521.
Valleys, formation of, 139, 144,
565 ; connection with faults, 626.
Valparaiso, raised beaches at, 68.
Veins, of lava, 21 ; of granite,
484 ; mineral, 370.
Venice, deep well at, 225.
Vertical succession, law of, 377.
Vesuvius, 20, 27 ; lava-streams
of, 30, 35.
Volcanoes, 20 ; connection with
earthquakes, 49 ; detrition of,
468 ; examples of, 35 ; extinct,
466 ; structure of, 20 ; sub¬
marine, 44.
Volcanic action, causes of, 32.
Volcanic bombs, 26.
Volcanic islands, 44.
Volcanic necks, 468.
Volcanic products, 25 ; agglome¬
rate, 26 ; ash, 26, 352 ; gases,
25 ; tuff, 28, 352.
Volcanic rocks, 91, 336, 465 ; pe¬
trology of, 481.
W.
■Wahsatch Mountains, 104.
Vales, North, cleavage in, 519 ;
geology of, 566, 575.
Wales, South, denudation of
608.
Wash, silting-up of the, 257.
666
INDEX.
AVater, action of hot, 395 ; circu¬
lation of, 108 ; erosive power
of, 136 ; minerals dissolved in,
114 ; solvent power of, 84 ;
underground action of, 113.
Waterfalls, 126, 587
Waterford, valleys of, 575.
Watersheds, 565.
Waves, power of, 164.
Weald, valleys of the, 580.
Weathering of rock surfaces, 91,
100.
West Point, U.S., 241.
Whin Sill, the, 492.
AVhite trap, 511, 533.
AMiittlesey Mere, marl of, 239.
Wight, Isle of, shape, 168 ; plat¬
form round, 594 ; structure of,
418.
AVilbraham, gravel at, 606.
Wind, action of, 98.
Winterbournes, 112.
Worms, mould made by, 209.
Wyoming, "bad lands" of, 97,
100.
Y.
Yale and Bala fault, 451.
Yare, valley of the, 77, 257.
Yarmouth, sands of, 257.
Yellowstone Park, geysers of,
205, 208.
Yorkshire, erosion of coast of,
172.
Z.
Zeolites, 323.
Zwitter-rock, 510.
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M.A., F.S.A.
BEVERLEY MINSTER. By Charles Hiatt.
WIMBORNE MINSTER and CHRISTCHURCH PRIORY. By the Rev. T
Perkins, M.A.
TEWKESBURY ABBEY AND DEERHURST PRIORY. By H. J. L. J. Massé, M.A.
WESTMINSTER ABBEY. By Charles Hiatt. [Vreparing.
' The volumes are handy in size, moderate in price, well illustrated, and written in a
scholarly spirit. The history of cathedral and city is intelli§;ently set forth and accompanied
by a descriptive survey of the building in all its detail. The illustrations are copious and well
selected, and the series bids fair to become an indispensable companion to the cathedral
tourist m England.'—Times.
'We have so frequently in these columns urged the want of cheap, well-illustrated and
well-written handbooks to our cathetirals, to take the place of the out-of-date publications of
local booksellers, that we are glad to .hear that they have been taken in hand by Messrs.
George Bell & Sons.'—St.Jamefs Gazette.
( 32 )
WEBSTER'S
INTERNATIONAL
DICTIONARY
OF THE ENGLISH LANGUAGE.
2If8 Pages. 3500 Illustrations.
PRICES:
Cloth, ll. Hi. 6a'. J half calf, 2/. 2i.; half russia, 2/. 5i.; full calf,
2/. 8i.; full russia, 2/. I2i.; half morocco, with Patent Marginal Index,
2/. Ss. ; full calf, with Marginal Index, 2/. I2i. Also bound in 2 vols.,
cloth, i/. 14J. ; half calf, 2/. I2i. ; half russia, 2/. i8i. ; full calf, 3/. y. ;
full russia, 3/. 15J.
The Appendices comprise a Pronouncing Gazetteer of the World,
Vocabularies of Scripture, Greek, Latin, and English Proper Names,
a Dictionary of the Noted Names of Fiction, a Brief History of the
English Language, a Dictionary of Foreign Quotations, Words, Phrases,
Proverbs, &c., a Biographical Dictionary with 10,000 names, &c., &c.
' We believe that, all things considered, this will be found to be the best
existing English dictionary in one volume. We do not know of any work
similar in size and price which can approach it in completeness of a vocabu¬
lary, variety of information, and general usefulness.'—Guardian.
' The most comprehensive and the most useful of its kind.'
National Observer.
'We recommend the New Webster to every man of business, every
father of a family, every teacher, and almost every student—to everybody,
in fact, who is likely to be posed at an unfamiliar at half-understood word or
phrase.' —;Sr. James's Gazette,
Prospectuses, with Specimen Pages, on Application.
THE ONLY AUTHORISED AND COMPLETE EDITION.
LONDON : GEORGE BELL àf SONS, YORK STREET,
COVENT GARDEN
30,000.—S. & S. z.oi.
550.2
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